updateref: Skip known-flaky test with Git v2.33.0

Git v2.33.0 had a bug in git-update-ref(1) where it didn't know to flush its output correctly. As a result we cannot be sure that references have been locked when calling updateref.Prepare() because we don't get a confirmation from Git. We have since upstreamed a fix for this bug which works as expected, but one of our tests is still frequently failing because of exactly that bug when running with Git v2.33.0.

Skip this test to reduce flakiness of our pipelines. We know it's an issue, we have a fix for it, and we want to upgrade the minimum required Git version anyway. So there is not much of a point to continue hitting this flake.