Ensure GEM_PATH always contains JRuby bundled gems

[Sharpie]

Short description

The Ruby standard library is split into two pieces:

• Default gems, which are installed directly into the $LOAD_PATH
  alongside core Ruby code.

• Bundled gems, which are installed into a repository that the
  Gem library looks at by default.

JRuby ships the latter inside jruby-stdlib.jar, which can be
referenced be referenced by the following Java classloader URI:

uri:classloader:/META-INF/jruby.home/lib/ruby/gems/shared

This commit updates the logic for setting GEM_PATH to always append
the classloader URI to the end of any path configured by the
application. This ensures that the full JRuby standard library is
available by default, and that any updates from new JRuby versions are
automatically taken up.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)