[CI] github action triggered on commit is currently broken

[IAlibay]

Whilst implementing concurrency groups in #3300, I didn't account for the fact that github.head_ref is only assigned in pull requests, so we unfortunately ended up with the following on merge: https://github.com/MDAnalysis/mdanalysis/actions/runs/840355785/workflow

Whilst it's not a massive impact on our current workflow (since we primarily rely on CI results whilst the PR is open), it does need a fixing.

Probably the best option here is to set the group to be unique on each PR, but the same for every commit. That way if you do several commits to develop then only the last one doesn't get cancelled.