Integrate Nix Cache

[dlipicar]

We're currently using DeterminateSystems/magic-nix-cache-action to cache our nix builds. However, the free no-FlakeHub-account mode that uses the Github Action cache is quite limited and we get rate-limited a lot when there's a lot of changes in the modules (eventually we get full caching after several builds).

Incorporate some better nix caching solution to the repo if possible.

The builds times are a bit bothersome for a couple releases until we stop getting throttled and everything gets cached properly, but it's not too terrible so this is not urgent.

[yakimant]

Options:

• https://nix-cache.status.im (self-hosted, used for most projects)
• https://www.cachix.org (used for one project, also free plan for OSS - 5GB)
• https://flakehub.com (no free plan)

[yakimant]

For self hosted would be nice to have:

• Github setup (actions & credentials)
• review garbage collection

[jakubgs]

What we want to cache is the result of nix build '.#lib' or something more?
How often do those change?
Could we get away with running just a nightly job for caching stuff?
Can this caching job run on Jenkins? I'd rather not share access to our cache with GitHub.