Nix CI on GitHub Actions

Every Nix build starts with an empty store and rebuilds the world.

Why it happens

/nix/store is not persisted and no binary cache is configured beyond the default.

What to change

1. Use a binary cache such as Cachix so derivations are fetched, not rebuilt
2. Cache /nix/store between runs where the runner permits it
3. Pin nixpkgs with a flake lock so the cache actually hits

Nix on runnerhut

jobs:
  build:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: cachix/install-nix-action@v30
      - uses: cachix/cachix-action@v15
        with: { name: acme }
      - run: nix build

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.