Re-bootstrap pipeline is failing for main

[ViktorHofer]

This isn't blocking at the moment. We performed the last re-bootstrap with a Preview4 build.

https://dev.azure.com/dnceng/internal/_build?definitionId=1371

The pipeline has a few assumption that don't hold anymore now:

• The VMR orchestrator content got moved from dotnet/sdk to dotnet/dotnet so the re-bootstrap PR should get opened against dotnet/dotnet.
• We don't tag VMR builds in main anymore.
• Source-build and UB should use the same versioning starting with Preview 5 so the pipeline can probably just look at the dotnet-unified-build's pipeline artifacts (at least soon?).

cc @dotnet/source-build

[ellahathaway]

These issues also apply to the release pipeline (both in the release stage and other stages as well). A number of adjustments will have to be made there.

[ellahathaway]

Draft PR - https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/49667