- 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- Ignore relevant RubyGems (and fix outdated comments)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`.