cachix: use the fancyful branch

Use the post-build hooks to upload to the cache at the end

.github/workflows/nix.yml

@@ -7,10 +7,8 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - uses: cachix/install-nix-action@v8
-    - uses: cachix/cachix-action@v5
+    - uses: numtide/cachix-action@only-cache
       with:
         name: nix-community
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
-        # Only needed for private caches
-        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
     - run: ./deploy --build-only

default.nix

@@ -1,14 +0,0 @@
-# Add derivations to be built from the cache to this file
-{ system ? builtins.currentSystem }:
-let
-  pkgs = import ./nix { inherit system; };
-
-  importNixOS = configuration: system:
-    (import "${toString pkgs.path}/nixos") {
-      inherit configuration system;
-    };
-in
-  pkgs.nix-community-infra // rec {
-    build01 = importNixOS ./build01/configuration.nix "x86_64-linux";
-    build01-system = build01.system;
-  }