Preview builds for PRs do not work from fork repos

[victorlin]

When I set up the preview builds using AWS Amplify, I was under the impression that previews would only show for PRs with a source branch in this repo, not from a fork: aws-amplify/amplify-hosting#508

However, the last comment on that issue states that forks should work as of late 2020, so it'd be worth trying this again.