[CI] update MDAnalysis/install-mdanalysis action #33

In PR #31 I pinned the MDAnalysis/install-mdanalysis action

to the version in PR MDAnalysis/install-mdanalysis#16 (because I wanted to test that it actually works in the wild and because I don't know how quickly that PR will get merged). Once the PR is merged, the pin here may fail or may be outdated and needs to be updated.