Set up CI (#9)

Very simple `nix-build` for the main build, the shell, and the Nixpkgs sanity check.

.github/workflows/main.yml

@@ -0,0 +1,19 @@
+name: CI
+on:
+  pull_request:
+    branches:
+      - main
+  push:
+    branches:
+      - main
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v26
+
+      - name: build
+        run: nix-build -A build -A shell -A checks