This PR adds a new GitHub action to commit build artifacts for Facebook
into a new protected branch. This will later be used to setup an
automatic sync to www.
The hacky spinloop is meant to be a temporary implementation until we
can support running a script internally on top of the synced diff
(coming soon). A GitHub token is otherwise required if we want to setup
a pub/sub between cirleci and github but it's not straightforward to
provision one for our org. So this workaround should do for now since we
won't keep it around for too long.
Example of it running and creating a commit on the `builds/facebook-www`
branch:
https://github.com/facebook/react/actions/runs/3516958576/jobs/5894251359
