Add build job to CI

.github/workflows/ci.yml

@@ -16,3 +16,21 @@ jobs:
       - uses: DeterminateSystems/nix-installer-action@main
       - uses: DeterminateSystems/magic-nix-cache-action@main
       - run: nix flake check
+  nix-build:
+    name: Nix Build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v30
+      - uses: cachix/cachix-action@v15
+        with:
+          name: x1e-nixos-config
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+      # Based on: https://carlosbecker.com/posts/github-actions-disk-space/
+      - run: |
+          sudo rm -rf /usr/share/dotnet /usr/local/lib/android /opt/ghc /opt/hostedtoolcache/CodeQL
+          sudo docker image prune --all --force
+          sudo docker builder prune -a
+      - run: |
+          nix build .#nixosConfigurations.iso.config.system.build.isoImage
+          sha256sum result/iso/cd.iso