Run stage 2 leg in VMR PR validation

[MichaelSimons]

In .NET 9.0 when all dependency flows are go directly into the VMR, I think we should consider running a stage 2 leg.

Stage 2 failures are the primary cause of source-build regressions. During Preview 6 and 7 there have been 6 different stage 2 regressions in source-build.

Source build PR validation currently takes ~50 minutes so running stage 1 and 2 theoretically would take 1 hr 40 mins. Not sure if this would be problematic with everything flowing directly into the VMR or not. Idealistically PR validation would be less than 1 hour. We could explore parallelizing the build and/or utilizing a build cache (more useful the higher the repo is in the dependency chain)

[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.

[mthalman]

Even if we don't end up adding an adding stage 2 leg to PR validation, we should consider defining a separate opt-in pipeline which does this scenario. There have been several times where people want to validate their changes in this stage 2 scenario. The steps for doing this manually is a bit onerous. So it would be convenient for people to have access to a pipeline to validate their changes in this scenario.

It wouldn't need to be triggered to run from a PR but could be used to manually run a build that targets a PR's changes. Perhaps there could be two pipelines: one based on the installer repo and one based directly on the dotnet repo. Depending on the change, it might be more convenient to use the dotnet repo as a way to test changes to avoid needing to create a patch via the installer repo.

[MichaelSimons]

We should block on #3608 before implementing this change.