Output MSBuild errors when publishing (#7566)

## Summary of changes

We saw the following error in our CI:

```
01:59:31 [INF] > "C:\Program Files\dotnet\dotnet.exe" publish c:\mnt\tracer\src\Datadog.Trace.MSBuild\Datadog.Trace.MSBuild.csproj --configuration Release --framework net461 --no-restore --no-build --output c:\mnt\shared\bin\monitoring-home\net461 /property:Platform=AnyCPU
01:59:34 [ERR] MSBUILD : error MSB4166: Child node "3" exited prematurely. Shutting down. Diagnostic information may be found in files in "C:\Users\ContainerAdministrator\AppData\Local\Temp\MSBuildTemp\" and will be named MSBuild_*.failure.txt. This location can be changed by setting the MSBUILDDEBUGPATH environment variable to a different directory.
01:59:34 [ERR] Target PublishManagedTracer has thrown an exception
```

In order to diagnose the actual error generated from MSBUILD, we are
checking for these MSBuild_*.failure.txt files and printing them into
the logs.

Potentially, these logs could be generated in any MSBUILD call. Since we
have only seen this in PublishManagedTracer, in this PR we are only
checking for error files in that task, but it could easily be added to
any other compilation or publishing task.

## Reason for change

## Implementation details

## Test coverage

We can force an intentional crash by using this target file and using
these properties: /p:CrashDuringBuild=true
/p:CustomBeforeMicrosoftCommonTargets="Crash.targets"

```
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <UsingTask TaskName="CrashNow"
             TaskFactory="RoslynCodeTaskFactory"
             AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll">
    <Task>
      <Using Namespace="System" />
      <Code Type="Fragment" Language="cs"><![CDATA[
        Environment.FailFast("Intentional crash for MSBuild failure-file test");
        return true; // not reached
      ]]></Code>
    </Task>
  </UsingTask>

  <!-- Fire before Build/Publish, but only when asked AND only for the target project -->
  <Target Name="CrashIfEnabled"
          BeforeTargets="Build;Publish;CoreCompile"
          Condition="'$(CrashDuringBuild)'=='true' and '$(MSBuildProjectName)'=='Datadog.Trace'">
    <Message Text="Crashing MSBuild worker for $(MSBuildProjectName)..." Importance="high" />
    <CrashNow />
  </Target>
</Project>
```

## Other details
<!-- Fixes #{issue} -->


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---------

Co-authored-by: Andrew Lock <[email protected]>

Author: NachoEchevarria

tracer/build/_build/Build.Steps.cs

@@ -52,6 +52,7 @@ partial class Build
     AbsolutePath WindowsSymbolsZip => ArtifactsDirectory / "windows-native-symbols.zip";
     AbsolutePath OsxTracerHomeZip => ArtifactsDirectory / "macOS-tracer-home.zip";
     AbsolutePath BuildDataDirectory => BuildArtifactsDirectory / "build_data";
+    AbsolutePath MsbuildDebugPath => TestLogsDirectory / "msbuild";
     AbsolutePath TestLogsDirectory => BuildDataDirectory / "logs";
     AbsolutePath ToolSourceDirectory => ToolSource ?? (OutputDirectory / "runnerTool");
     AbsolutePath ToolInstallDirectory => ToolDestination ?? (ToolSourceDirectory / "install");
@@ -794,17 +795,27 @@ async Task DownloadWafVersion(string libddwafVersion = null, string uncompressFo
                 ? TargetFrameworks
                 : TargetFrameworks.Where(framework => !framework.ToString().StartsWith("net4"));
 
-            // Publish Datadog.Trace.MSBuild which includes Datadog.Trace
-            DotNetPublish(s => s
-                .SetProject(Solution.GetProject(Projects.DatadogTraceMsBuild))
-                .SetConfiguration(BuildConfiguration)
-                .SetTargetPlatformAnyCPU()
-                .EnableNoBuild()
-                .EnableNoRestore()
-                .CombineWith(targetFrameworks, (p, framework) => p
-                    .SetFramework(framework)
-                    .SetOutput(MonitoringHomeDirectory / framework))
-            );
+            try
+            {
+                // Publish Datadog.Trace.MSBuild which includes Datadog.Trace
+                DotNetPublish(s => s
+                    .SetProject(Solution.GetProject(Projects.DatadogTraceMsBuild))
+                    .SetConfiguration(BuildConfiguration)
+                    .SetTargetPlatformAnyCPU()
+                    .EnableNoBuild()
+                    .EnableNoRestore()
+                    .CombineWith(targetFrameworks, (p, framework) => p
+                        .SetFramework(framework)
+                        .SetOutput(MonitoringHomeDirectory / framework)
+                    .SetProcessEnvironmentVariable("MSBUILDDEBUGPATH", MsbuildDebugPath))
+                );
+            }
+            catch
+            {
+                // Print tail of MSBuild failure notes, if any, then rethrow
+                MSBuildLogHelper.DumpMsBuildChildFailures(MsbuildDebugPath);
+                throw;
+            }
         });
 
     Target PublishManagedTracerR2R => _ => _

tracer/build/_build/MSBuildLogHelper.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using Nuke.Common;
+using Nuke.Common.IO;
+
+using Logger = Serilog.Log;
+
+internal static class MSBuildLogHelper
+{
+    internal static void DumpMsBuildChildFailures(string msbuildDebugPath, int tailChars = 10 * 1024, int maxFiles = 2)
+    {
+        try
+        {
+            if (!Directory.Exists(msbuildDebugPath))
+            {
+                Logger.Information($"No MSBuild failure directory: {msbuildDebugPath}");
+                return;
+            }
+
+            var files = Directory.EnumerateFiles(msbuildDebugPath, "MSBuild_*.failure.txt", SearchOption.AllDirectories)
+                .OrderByDescending(File.GetLastWriteTimeUtc)
+                .Take(maxFiles)
+                .ToList();
+
+            if (files.Count == 0)
+            {
+                Logger.Information("No MSBuild failure notes found.");
+                return;
+            }
+
+            foreach (var file in files)
+            {
+                try
+                {
+                    // Use BOM detection: handles UTF-8/UTF-16 automatically
+                    using var fs = new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+                    using var sr = new StreamReader(fs, Encoding.UTF8, detectEncodingFromByteOrderMarks: true);
+                    var text = sr.ReadToEnd();
+
+                    var tail = text.Length > tailChars ? text[^tailChars..] : text;
+
+                    Logger.Error($"----- BEGIN {file} (showing last {tail.Length} of {text.Length} chars) -----");
+                    Logger.Error(tail);
+                    Logger.Error("----- END -----");
+                }
+                catch (Exception exRead)
+                {
+                    Logger.Warning($"Could not read {file}: {exRead.Message}");
+                }
+            }
+        }
+        catch (Exception ex)
+        {
+            Logger.Warning($"Failed to dump MSBuild child-node diagnostics: {ex.Message}");
+        }
+    }
+}