ci(zen-update): add job to test builds

.github/workflows/zen-update.yml

@@ -24,6 +24,7 @@ jobs:
 
       - name: Install Nix
         if: steps.check.outputs.should_update == 'true'
+        # TODO: they're @ v31 so next time let's try DeterminateSystems/nix-installer-action@main
         uses: cachix/install-nix-action@v27
         with:
           nix_path: nixpkgs=channel:nixpkgs-unstable
@@ -41,8 +42,35 @@ jobs:
           .github/update.sh --ci
 
       - name: Commit changes
+        id: commit
         if: steps.check.outputs.should_update == 'true'
         uses: stefanzweifel/git-auto-commit-action@v5
         with:
           commit_message: "${{ steps.update.outputs.commit_message }}"
           file_pattern: "*"
+    outputs:
+      success: ${{ steps.update.outputs.should_update }}
+
+  test-build:
+    if: needs.update-zen-browser.outputs.success == 'true'
+    continue-on-error: false
+    name: Test package build
+    runs-on: ubuntu-latest
+    needs: update-zen-browser
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixpkgs-unstable
+
+      - name: Setup Nix Magic Cache
+        uses: DeterminateSystems/flakehub-cache-action@main
+
+      - name: Build packages
+        run: |
+          for package in $(nix flake show --json 2>/dev/null | jq -r '.packages.["x86_64-linux"] | keys | .[]'); do
+              nix build --system 'x86_64-linux' ".#$package"
+          done