add cachix workflow

.github/workflows/cachix.yml

@@ -0,0 +1,21 @@
+name: "Test"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v14
+      with:
+        name: mycache
+        # If you chose signing key for write access
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+        # If you chose API tokens for write access OR if you have a private cache
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix-build
+    - run: nix-shell --run "echo OK"