Need a "sync-master" workflow

[jasonkarns]

Would like to transition all the bats-core repos to use main instead of master default branch. I've had success with nodenv by using a workflow to simply force-push master whenever main is updated. This gives us a transition period where (presumably) existing clones do not break, but fresh clones use main. Then over time, the number of old clones using master will diminish and we can eventually delete the branch.