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

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 https://github.com/dotnet/source-build/issues/5198 which worked in a PR build but failed in the official build.