Allow pull requests from forks to pass

Some GitHub actions fail when PRs are created from a forked repo. This prevents external contributors from making contributions to the repo. I wasn't able to fix the issue after several attempts (see fdc1b8e and 0d296b9) which only partly fixed the issue. I'm filing this to track that we still need to make changes to our GH actions to unblock external contributions.

See #198 as an example. 