ci: add magic nix cache

[sagikazarmark]

Overview

Fixes #105

Notes for reviewer

[ramizpolic]

I don't see much difference in pipeline steps with and without caching, is this maybe because this was the first execution or perhaps that it's not present on main branch, @sagikazarmark ?

[sagikazarmark]

Main branch (without caching): https://github.com/bank-vaults/vault-operator/actions/workflows/ci.yaml?query=branch%3Amain

Cache branch: https://github.com/bank-vaults/vault-operator/actions/workflows/ci.yaml?query=branch%3Anix-cache

Test job without caching: ~7 minutes
Test job with Nix caching only: ~3 minutes
Test job with Nix+Go caching only: no real improvement

[ramizpolic]

Great improvements 🚀

[akijakya]

🥳

[sagikazarmark]

Managed to improve things further by segmenting the Go cache for each job. It takes up more space in the cache, but different jobs download different dependencies (eg. build doesn't download test dependencies, etc)