Similar to dotnet/msbuild#13124 we want an end-to-end correctness/reliability test that we can build dotnet in MSBuild's new -mt (MultiThreaded) mode which is actively developed across repositories (it requires migrations of MSBuild tasks). Currently we will need to exclude some projects from -mt build, as there are blockers tracked in MSBuild repo.

This will enable better validation of MSBuild task migration to the new mode.

Once the feature is stable the intent is to switch to using it by default for all repos, but right now that is not feasible.

1. identify which repos are not buildable
2. figure out how to inject -mt parameter for msbuild in repo builds
3. create the CI job with exclusions