Change update-response generation script to create one commit per OS+arch tuple

As part of tor-browser#43468 (closed) we've rolled back the macOS-specific update responses ( tor-browser-update-responses@0caa0c54). This would have been a bit less stressful if I could have just reverted a particular macOS-specific commit rather than surgically+manually reverting portions of the 14.0.5 commit.