GH Actions: Use cache

[JoshKeegan]

Currently GH Actions pipeline is WIP. Artefacts are being used to share artefacts between jobs, but for short-lived things that don't need to be accessed after the pipeline is done cache is probably better. Needs more reading but see https://levelup.gitconnected.com/github-actions-how-to-share-data-between-jobs-fc1547defc3e

[JoshKeegan]

Looking (albeit quickly) at the docs it looks like cache is a genuine cache. Whilst there is branch isolation I'm not sure there is build isolation. Needs more reading but I don't think it's as clear-cut as that article makes it seem and artefacts are probably fine for what we're doing.