CI: Run Sdist less often(only on master/nightlies and PRs labeled build)

[lithomas1]

We might want to consider running the sdist checks less. They take around 10-15 minutes each and personally, I have only seen them fail once or twice ever(which were on PRs that changed build stuff).

We could try moving them to only run on PRs that are labeled "Build" and on merged commits first, and then maybe look to move them to nightly.

cc @fangchenli @simonjayhawkins

[simonjayhawkins]

I think OK to only run on commits to master only as a first step. This should still provide more visibility of issues than the nightly builds.