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