Test flakiness in multithreaded bootstrap tests

check out https://dev.azure.com/dnceng-public/public/_build/results?buildId=1416074&view=ms.vss-test-web.build-test-results-tab&runId=39390610&resultId=100662&paneView=debug

In this build, the test failed with (condensed):

```
Plain Text
error : You must install or update .NET to run this application.
error :
error : App: D:\a_work\1\s.dotnet\sdk\10.0.107\Roslyn\bincore\csc.exe
error : Architecture: x64
error : Framework: 'Microsoft.NETCore.App', version '10.0.7' (x64)
error : .NET location: D:\a_work\1\s\artifacts\bin\bootstrap\core
error :
error : The following frameworks were found:
error : 10.0.6 at [D:\a_work\1\s\artifacts\bin\bootstrap\core\shared\Microsoft.NETCore.App]
```

that makes sense to me! we [updated to 10.0.7 in an arcade update](https://github.com/dotnet/msbuild/pull/13721/changes#diff-8df3cd354bc584349d04ad5675b33c042d8b99b741b8b95af394c55e0f5001bfR13) but [didn't update the bootstrap version](https://github.com/dotnet/msbuild/blob/5dee75f0d635ee0c63dad6738e4191bab69b09b7/eng/Versions.props#L84).

HOWEVER!!! Only one of the tests failed. Shouldn't they all? And also, if we're calling the bootstrap output, which is my read of the test, how did it come to be that we're running csc.exe from the Arcade-delivered .dotnet folder?

Is it a problem with state leaking somehow or what? I might also suggest capturing the build logging in the test even on success--right now it's only on failure.