I began by experimenting with "except" but then discovered that
that's in danger of being deprecated in favour of workflow:rules.
Which led to 7bed6e5a but I forgot to remove these lines.

We disallow direct pushes to master, so all commits on master are just
merge commits from another branch. They will already have had the CI
tests run on them on their feature branch, so there's no point running
the tests again when we're simply merging into master.