[sbrp] Enable VMR Verification pipeline in PRs

[NikolaMilosavljevic]

Enable VMR Verification pipeline in source-build-reference-packages PRs.

New pipeline, for this repo, should only run SB legs.

[NikolaMilosavljevic]

To enable this, with a scoped-down list of verification legs, we'd need to add a way to specify build intent for the vmr-build stage.

In the past we had isSourceOnlyBuild variable and template parameter: https://github.com/dotnet/sdk/blob/9dee3cf508254bc3859d9cd2fe96aeb2ad1b9751/eng/pipelines/vmr-build-pr.yml#L75

We conditioned its value based on the pipeline name, i.e. if it ended with -source-build. Build stages were conditioned bases on this parameter: https://github.com/dotnet/sdk/blob/9dee3cf508254bc3859d9cd2fe96aeb2ad1b9751/eng/pipelines/templates/stages/vmr-build.yml#L81-L83

The proposal is to add isSourceOnlyBuild parameter to https://github.com/dotnet/arcade/blob/main/eng/common/templates/vmr-build-pr.yml. Pass the parameter to the build stage and use it to condition individual build jobs or templates.

This will address the needs for this specific case - SBRP repo. It will not address some other cases, i.e. skipping Linux builds for winforms repo builds. I believe those cases should be addressed separately.