Fix unvendored RubyGems

[MikeMcQuaid]

• Avoid caching RubyGems in GitHub Actions tests.yml to catch this in
  future.
• Run brew doctor in GitHub Actions tests.yml after installing
  RubyGems to catch this in future.
• Ignore relevant RubyGems (and fix outdated comments)
• Never auto-require RubyGems that aren't vendored.
• Update bundler/setup.rb.

Alternate approach to #8508
Fixes #8503 (comment)

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