Automate building new releases

[eregon]

For CRuby we can use:
https://github.com/ruby/www.ruby-lang.org/blob/master/_data/releases.yml
For JRuby & TruffleRuby we could query GitHub releases.

Or maybe easier, since anyway we need it in ruby-build, we could check https://github.com/rbenv/ruby-build/tree/master/share/ruby-build / ruby-build -l / ruby-build --definitions

In both cases we'd need to build only the releases which are not already built.

This job would be a scheduled daily job on either setup-ruby or ruby-builder.
It would detect the new release(s) and for each commit and push to ruby-builder.
That would then build the new releases on all platforms (except Windows for CRuby).
On success that job could maybe also automatically create a PR to setup-ruby to add it.

Based on #240 (comment)

[eregon]

There is a new workflow, https://github.com/ruby/ruby-builder/blob/master/.github/workflows/check-new-releases.yml
which automatically builds new releases as soon as they are in https://github.com/rbenv/ruby-build master.

Example commit created by that: ruby/ruby-builder@7215ba5
(It's a separate bot account for enhanced security, so it only has the necessary accesses).

[eregon]

All done now.

check-new-releases.yml checks daily if there are new releases known in ruby-build, if so, it pushes a commit to itself, which triggers another workflow which builds the release on all platforms and then automatically creates a PR to setup-ruby!

check-new-releases.yml can also be triggered manually (to not wait for the cron job), as well as the automated creation of the PR (for a given version).