Symbols missing in source-build

[ViktorHofer]

Currently emitted as a warning:

Packaged all symbols in '/__w/1/s/artifacts/assets/Release/dotnet-symbols-all-10.0.100-preview.6-centos.9-x64.tar.gz'
/__w/1/s/eng/finish-source-only.proj(227,5): warning : Did not find PDBs for the following SDK files:
/__w/1/s/eng/finish-source-only.proj(227,5): warning : sdk/10.0.100-ci/ref/Microsoft.Build.Framework.dll
/__w/1/s/eng/finish-source-only.proj(227,5): warning : sdk/10.0.100-ci/ref/Microsoft.Build.Utilities.Core.dll

Example build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1051585&view=logs&j=609589e2-4f74-5576-cdb7-914bcaea778b&t=8309e960-3e55-5314-da53-c9e47ab6c0d0

cc @dotnet/source-build

[mthalman]

This is caused by dotnet/roslyn#78721

[mthalman]

Let's use this issue as both a tracking issue for dotnet/roslyn#78721 as well as follow-up work to remove the workaround here:
https://github.com/dotnet/dotnet/blob/57b0396ae0d21b9f0dfe0d208c57822fb88f9a8d/test/Microsoft.DotNet.SourceBuild.Tests/SymbolsTests.cs#L47-L55