Particularly the `bundler` Gem requires the right Ruby binary to be in
the PATH to work correctly if the Ruby binary used to run Homebrew is
overridden via `HOMEBREW_RUBY_PATH` and thus intentionally not prefixed
to the PATH (as that would affect formulae with a Ruby dependency).
The workaround here is to prefix this path only when a Gem is activated
via `Homebrew.install_gem_setup_path!` (currently used in `brew man`,
`brew style`, and `brew tests`).
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