Draft: 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.