feat(nix-based): build cache

[tulilirockz]

We gotta implement some kind of build cache for this project, because re-fetching everything for every image build is not particularly efficient.

We can do that by using one of these approaches:

• Nix Magic Cache by Determinate Systems
• Layered Nix Stores (mounting these things require root, so maybe not)
• Chroot Nix Stores