Allow auto-rebuilds of merged branches in GitHub

[Shadowfiend]

Right now if we want to make sure that a PR's merge with main runs against CI after a different PR is merged, we have to block GitHub merges on re-merging main into the PR. This is lame.

Ideally Heimdall would monitor main merges and trigger rebuilds on the pr/number/merge HEAD that GitHub provides associated with the appropriate PR, so we don't have to do explicit merge commits but have confidence with the remerge. Suspicion is this is possible with the GitHub API, but it's not a guarantee.