Use the actual JRUBY_REVISION in release note

[ntkme]

Currently the github release note has the "latest commit" of jruby, however, it's not always the same commit of the latest maven snapshot due to the schedule of jruby nightly builds.

For example, I was downloading a build from https://github.com/ruby/jruby-dev-builder/releases/tag/v20250218.190130 - The release note says v20250218.190130 has jruby 00021bc2d8067eb2b7478352a7883b70b9069246, but in reality it was f1f9396cd79bb66b79217ce6e3c894ae5a59934d.

This PR fix the release note by actually checking the JRUBY_REVISION on the downloaded jruby maven snapshot, and then set that as the release note.

[ntkme]

Did some refactoring:

• Change the release note to jruby/jruby@${commit} ${maven_download_url}, and run uniqueness check against the download url instead of commit.
• Remove unused code.
• Cache nokogiri gem with setup-ruby.

[eregon]

This looks great, thank you!

I'm not sure it's worth caching the install of nokogiri since it's precompiled these days, but it seems fine (a bit more complexity with moving the test Gemfile).