Add NixOS to build step as well

.github/workflows/build.yml

@@ -31,6 +31,10 @@ jobs:
         hostname: ${{ fromJSON(needs.determine_hosts.outputs.hosts) }}
 
     steps:
+      - uses: actions/checkout@v5
+      - uses: https://github.com/cachix/install-nix-action@v31
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
       - name: "Define host"
         env:
           hostname: ${{ matrix.hostname }}