Add GitHub CI

Author: neilmayhew

.github/workflows/nix-build.yml

@@ -0,0 +1,25 @@
+name: Nix Build
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+      fail-fast: false
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v31
+
+    - name: Build
+      run: nix build