Use .NET 5 preview in release pipeline

Author: brcrista

.azure/pipelines/publish.yml

@@ -5,6 +5,12 @@ pool:
   vmImage: ubuntu-latest
 
 steps:
+- task: UseDotNet@2
+  displayName: 'Install .NET Core SDK'
+  inputs:
+    version: '5.0.100-rc.1.20452.10'
+    installationPath: $(Agent.ToolsDirectory)/dotnet
+
 - script: dotnet pack --configuration Release
   displayName: dotnet pack