Setup Github CI.

.github/workflows/build.yml

@@ -0,0 +1,18 @@
+name: "Build"
+on:
+  pull_request:
+    branches:
+    - master_jammy
+  push:
+    branches:
+    - master_jammy
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v18
+      with:
+        nix_path: nixpkgs=channel:nixpkgs-unstable
+    - run: nix build
+    - run: nix flake check