Add `SOURCE_REF_TO_BUILD` rather than manual input field?

I think it's simpler to stay close to numpy while we are still getting to know this new release procedure.

I can see a few advantages to having SOURCE_REF_TO_BUILD in the yaml rather than a manual input field:

• harder to mess up, it's set once in a release branch rather than having to type it manually a few times
• audatibility. Right now nothing in the log is telling you what the input field was. If it's in the repo and an environment variable it's probably in the CI log.

I could not find more details by looking in numpy and numpy-release. The context around not using it in scikit-learn is in https://github.com/scikit-learn/scikit-learn/issues/34320#issuecomment-4752144202.

Also once it is set in a release branch you don't have to think too much about it, rather than typing a few times during the release process.

I think we could also set SOURCE_BUILD_REF to refs/pull/<pr_number>/merge as we did if we want to test on a scikit-learn PR, which did for 1.9.1 since this is part of our release process.

Note that numpy has an issue about this https://github.com/numpy/numpy-release/issues/13, not sure if I am missing something or they haven't realized they can set it to a PR merge ref.

Start by reviewing the release procedure and its current manual input handling, then compare the proposed SOURCE_REF_TO_BUILD setting with the linked NumPy and scikit-learn discussions. Determine how a release branch or pull-request merge ref would be represented in the existing YAML and CI logs; done means the release flow no longer requires repeated manual source-reference entry and remains auditable.