Make signature verification message display only once globally

Changed _validationMessagesDisplayed flag from instance-level to static to ensure
"Skipping NuGet package signature verification." message appears only once across
all NuGetPackageDownloader instances during workload operations, rather than once
per instance.

Co-authored-by: marcpopMSFT <[email protected]>

Author: Copilot

src/Cli/dotnet/NugetPackageDownloader/NuGetPackageDownloader.cs

@@ -39,7 +39,7 @@ internal class NuGetPackageDownloader : INuGetPackageDownloader
     /// </summary>
     private readonly IReporter _reporter;
     private readonly IFirstPartyNuGetPackageSigningVerifier _firstPartyNuGetPackageSigningVerifier;
-    private bool _validationMessagesDisplayed = false;
+    private static bool _validationMessagesDisplayed = false;
     private readonly ConcurrentDictionary<PackageSource, SourceRepository> _sourceRepositories;
     private readonly bool _shouldUsePackageSourceMapping;