workflows/lint: fully skip the `commits` job in Merge Queues
Follow-up to 7cf59724104ef43e73e96176acbb8a5be79b1921
While the JS script already returned early, we can save a few resources
by skipping the job entirely when there's no `pull_request` context.
(cherry picked from commit 7cfe9b8c8531a6b6d97fbaaed20779d594ab1e76)