[tests] Ignore output that looks like errors when installing .NET on Windows. (#21430)

Just trust the exit code instead.

This way the installation doesn't fail due to random output.

tests/dotnet/Windows/InstallDotNet.csproj

@@ -171,6 +171,7 @@
         Command=""$(DotNetToolPath)" workload install %(_WorkloadIds.Identity) --skip-manifest-update --verbosity diag --temp-dir "$(DotNetTempDirectory)" --configfile "$(MacIosRootDirectory)NuGet.config" --skip-sign-check"
         WorkingDirectory="$(MSBuildThisFileDirectory)"
         EnvironmentVariables="DOTNET_MULTILEVEL_LOOKUP=0"
+        IgnoreStandardErrorWarningFormat="true"
     />
     <Touch Files="$(DotNetPacksDirectory).stamp" AlwaysCreate="true" />
   </Target>