Priority annotation sometimes ignored on queued root-ish tasks

[gjoseph92]

See https://github.com/dask/distributed/pull/6614/files#r949738364. If a queued task has a user-defined priority, a downstream task may be scheduled ahead of it because of the order recommendations are created. We could sort the recommendations, but since this would be a pretty uncommon case, it's nice to avoid the sort in the common case.