Run new PHP integration tests in CI

Author: john-shaffer

.github/workflows/build.yml

@@ -29,6 +29,10 @@ jobs:
       - name: Install Nix
         uses: DeterminateSystems/determinate-nix-action@v3
 
+      - name: Run tests
+        run: nix flake check
+        working-directory: dev
+
       - name: Build plugin zip
         run: |
           nix build .#wp2static