campaigns: cancelled GitLab pipelines don't take check state out of pending

Created by: LawnGnome

Originally posted by @eseliger in https://github.com/sourcegraph/sourcegraph/pull/12139#issuecomment-667984395

I won't reproduce the full comment here, but basically: a cancelled pipeline was synced and didn't change the check state from pending to whatever is appropriate (probably unknown). I need to investigate the offending pipeline and merge request in the API to ensure we're doing the right thing.