Setting the environment variable `HOMEBREW_RUBY_PATH` allows one to run
Homebrew with a non-standard Ruby, even one that is not in `PATH`. This
creates the problem that the `gem` in `PATH` might not be the right one
to call, possibly leading to confusing results as the wrong RubyGems
installation is queried/manipulated.
ClosesHomebrew/homebrew#46185.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>