install-bundler-gems: change_privilege when necessary.

This ensures that gems are able to be installed when using e.g.
`ruby -I` in their installation scripts.

Library/Homebrew/dev-cmd/install-bundler-gems.rb

@@ -32,6 +32,7 @@ module Homebrew
           Homebrew.forget_user_gem_groups!
         end
 
+        Process::UID.change_privilege(Process.euid) if Process.euid != Process.uid
         Homebrew.install_bundler_gems!(groups:)
       end
     end