Previous attempt to use a full path when the running version is
different than the one that would be activated by default are different
was not correct.

This is because if this error happens in a `bundle exec` context, the
`Gem.loaded_specs` is cleared because we're in an exec'd process, so
will be always using a full path in these cases.

This alternative approach should do what I was expecting.

https://github.com/rubygems/rubygems/commit/e0bb8bf51b