Cache go modules and build cache

[MarcoPolo]

Ideally this should speed up:

1. Fetching dependencies
2. Building
3. Testing (?)

[galargh]

Yes! That's a great idea. I'm not sure why we never got around to it. I think we should add it ahead of Go 1.21 release.

[galargh]

This issue was transferred to ipdxco/unified-github-workflows#46 in preparation for archiving of this repository. FYI, it was blocked on setup-go incorrect setup of caches on Windows machine. I fixed it in actions/setup-go#411. We'll be able to proceed with it once a release with that fix is out.