build(cachix): compile and cache built binaries for faster installs

[zimeg]

• https://docs.cachix.org/getting-started
• https://github.com/nix-community/nur-packages-template/blob/06c636d7be402749f22c8df01aad726901ef310b/.github/workflows/build.yml#L53-L60
• https://github.com/cachix/cachix-action
• https://nix.dev/tutorials/nixos/continuous-integration-github-actions