ci: setup cachix for flake package outputs

.github/workflows/cachix.yaml

@@ -0,0 +1,23 @@
+name: "Build packages for cachix"
+on:
+  push:
+    paths:
+      - 'pkgs/**'
+      - 'flake.nix'
+      - 'flake.lock'
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v13
+      with:
+        install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
+        install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+    - uses: cachix/cachix-action@v10
+      with:
+        name: alarsyo
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: .github/workflows/build-flake-packages.sh