Cache builds in GitHub actions

[kasbah]

This reduces time to deploy our previews by only building what is needed. These builds tend to take 1-2 minutes but I have also noticed frequent network related intermittent failures with e.g. Math docs that this will avoid on unrelated PRs.

Downside is we need to keep the cache key computation for frontend updated when changing any in-repo dependencies but I think it's worth it.

[kasbah]

➡️

when cached

[epatters]

Thanks Kaspar! I'm a little nervous about the cache invalidation but I've also been annoyed by the intermittent failures in GitHub actions, especially when building unrelated artifacts, so I'm happy to give this a try.