Ci skip master

There's no point running the automated CI tests on commits to master: because of how we have the repository configured, all manual commits are to a branch (where we want the test suite to run) and so the commits to master are only ever merge commits.

(Whilst there's no harm in re-running the tests there's no point: I've just set the repository to require the test pipeline to succeed before MRs are permitted, so it is actually impossible for any tests on merge-to-master to fail!)