Setup GitHub action for CI.

Author: wsx-antithesis

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: "CI"
+on:
+  pull_request:
+    branches:
+    - main
+  push:
+    branches:
+    - main
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v22
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - run: nix flake check