Implement test source-only branch test sync and build on installer PRs

[mmitche]

When a PR is made to installer, the VMR syncs and runs a test today. This test is of the source-build functionality, and optionally the UB functionality. This sync process should sync a test main VMR branch and run UB (non-source only builds) tests, and merge to a test source-only branch from this main VMR branch (applying the cloaking process) and run the source-only tests.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mmitche]

Going with a different solution in #4088