Use SB validation jobs in PR builds the same way as official builds

[mthalman]

Today SB jobs in PR builds run tests in the same job that the build occurs. But in official builds, the tests run in a separate job because it also requires artifacts from other jobs. We should follow the same pattern of a separate validation job for PR builds as well, even though it only requires the output of the SB job.

This will make the functionality of the pipeline more consistent. It will also prevent issues like #5198 which worked in a PR build but failed in the official build.

[MichaelSimons]

[Triage] One concern here is that with the work to support repo level VMR/SB validation legs is sensitive to the overall time to run the SB legs. We want to keep this to a minimum. It may be the case that we should continue to run build/validation together in these repo scenarios. The main problem they would face is the delay in acquiring a build agent to run the validation stage. In the VMR validation, this is not a concern because the vertical legs are longer.