If someone pushes to autoland with DONTBUILD in the commit message, builds and tests won't be scheduled. Presumably this is because the change is trivial.

However if the push would have been a backstop push (a push would otherwise have run everything to make sure we didn't miss any regressions), it means the backstop is wasted and we need to wait another 10 pushes for the next one. This makes it harder for sheriffs as there are fewer merge candidates and regression ranges are bigger.

With https://bugzilla.mozilla.org/show_bug.cgi?id=1660506 being solved, there's now an index we can use to find the latest backstop push. Rather than ignoring DONTBUILD, this means we could bump the backstop back a push. We could decide whether or not to keep these going based off of push id, or whether we can allow them to digress (if we do the latter, we'd need to also fix expanded pushes to do the same).