pr-auditor: check on synchronize

Created by: bobheadxi

As a required check, pr-auditor's check status messes up if a subsequent push is made to a branch. It appears we can't set a ref instead of a specific SHA, so I guess we have to run on pushes.

I think this is what the synchronize event does: https://github.community/t/what-is-a-pull-request-synchronize-event/14784

Test plan

Open PR, pre-merge passes.

Push to PR, pre-merge still marked as green.