CI: add nix package build test

Co-authored-by: Edouard Choinière <27212526+echoix@users.noreply.github.com>

.github/workflows/test-nix-package.yml

@@ -0,0 +1,25 @@
+---
+name: Test Nix package
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+  cancel-in-progress: true
+
+jobs:
+  test-package:
+    runs-on: ubuntu-22.04
+
+    steps:
+      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      - name: Install nix
+        uses: DeterminateSystems/nix-installer-action@main
+
+      - name: Build all packages
+        run: nix build -L --accept-flake-config .#grass