Allow running `build-release.yml` against a branch

[hugovk]

Currently, .github/workflows/build-release.yml takes a tag number:

https://github.com/python/release-tools/blob/9721c249ff1de736fcf180c4a806ce75372ebf2a/.github/workflows/build-release.yml#L35-L37

Which can be used as the input when triggering a run:

https://github.com/python/release-tools/actions/workflows/build-release.yml

It would be useful if we could also give a branch name, so for example we can trigger a run against upstream main or 3.14, so we can verify the latest state will build.

This will also help with PRs like https://github.com/python/release-tools/pull/284 where we specify which arguments to run against the Android test runner, and the test runner is in the upstream repo.

We'll need to take care of v prefix that is currently prepended in some places.