Automate stable and pretest builds

[jimeh]

Right now I need to manually trigger the build GitHub Actions workflow with the
relevant git tag ref to create a stable or pretest build.

This should be automated by checking the list of git tags in the
emacs-mirror/emacs repository nightly, and automatically triggering builds
for any new stable or pretest builds.

For each matching tag found, I think it would be reasonable to simply check if
the expected GitHub release exists for it, and if not, trigger the workflow.

A minimum version for stable and pretest builds should also set, as I don't
believe there's any point in building anything before Emacs 27.1.