Add a GitHub CI workflow for testing the nix flake

Author: neilmayhew

.github/workflows/nix-build.yml

@@ -0,0 +1,36 @@
+name: Nix Build
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Install nix
+      uses: cachix/install-nix-action@v31
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+
+    - name: Configure nix
+      run: echo 'accept-flake-config = true' >>~/.config/nix/nix.conf
+
+    - name: Check nix build
+      run: nix build
+
+    - name: Check nix develop
+      run: |
+        SYSTEM=$(nix eval --impure --expr builtins.currentSystem)
+        SHELLS=$(nix eval --raw ".#devShells.$SYSTEM" \
+          --apply 's: builtins.toString (builtins.attrNames s)')
+        for NAME in $SHELLS
+        do
+          echo ">>>> $NAME <<<<"
+          nix develop ".#$NAME" -c true
+        done