CI: try and cache the bundle between builds

[joshpencheon]

It currently takes ~5 minutes of compute time per job to do this.

This PR uses caching functionality provided by the default setup-ruby action to preserve a cache of the installed + built gems between runs.

[joshpencheon]

I'll restart the builds once done, to see what effect there is.

[joshpencheon]

Although the caching worked, it would appear that the version of sassc being used made use of some very CPU-specific features during the native install. The upshot being that the bundle cache key was not granular enough, and builds using the cache could fail depending on the CPU they were scheduled against. More recent sassc versions apparently build in a slightly more flexible way.

[joshpencheon]

It would seem the "Ruby + Bundle" drops to <10s with maximal use of the cache. Even if there are some changes to the bundle in a commit, the unchanged portion still benefits from the cache too. :)

[ollietulloch]

Thanks @joshpencheon - looks good, now seeing checks (eg Test / brakeman (push)) in ~25 seconds instead of ~5 minutes