gitlab-ci: Re-enable builds for merge requests.
When the whitespace job was added, we marked it with only: merge_requests because we wanted it to only run on new merge requests. However, adding an `only` clause like this disables all the unmarked jobs for merge requests, which means we don't get the more important feedback about build failures. Instead, use the `rules` key, added in Gitlab 12.3. This more flexible mechanism is currently recommended and lets us specify what we want without having to add additional decorations to every other job.