ci: Build flake outputs on self-hosted runner

Same as nammayatri/nammayatri#8494

.github/workflows/nix.yml

@@ -0,0 +1,28 @@
+name: CI
+
+on:
+  # Triggers the workflow on push or pull request events but only for the "main" branch
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  build:
+    runs-on: x86_64-linux
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/cachix-action@v15
+        with:
+          name: nammayatri
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+          skipPush: true
+
+      - name: Build all flake outputs
+        run: om ci