CI: Restore mocha-github-actions-reporter

[voxpelli]

Removed mocha-github-actions-reporter because it was being installed within the workflow rather than being a dev dependency of the package.json, which makes versioning of it non-straightforward.

And felt a bit odd to have all the other kinds of reporters provided within the module but not this – if we need it then so do others, we should dogfood it (and possibly offer to upstream it: daniellockyer/mocha-github-actions-reporter#7 (comment))

Originally posted by @voxpelli in #5177 (comment)