[rubygems/rubygems] Fix `bundle exec rake install` failing when local gem has extensions and `gemspec` DSL is being used

In a `bundle exec` context, the local specification will actually be
part of the known specifications, so RubyGems will assume it has already
been installed, which is not actually true.

This will cause `RequestSet` to rebuild extensions for a gem that's not
actually installed, causing errors.

The fix is to make sure detection of installed activation requests
considers not only that there's a known spec with the same full name as
the one being installed, but also that this spec is installed in the
same gem_home were pretend to install the new gem.

https://github.com/rubygems/rubygems/commit/a8ef1286a6