ci: colorscheme update should verify nix hash and build

.github/workflows/update-colorschemes.yml

@@ -49,8 +49,14 @@ jobs:
           # Only proceed if build.zig.zon has changed
           if ! git diff --exit-code build.zig.zon; then
             nix develop -c ./nix/build-support/check-zig-cache-hash.sh --update
+            nix develop -c ./nix/build-support/check-zig-cache-hash.sh
           fi
 
+      # Verify the build still works. We choose an arbitrary build type
+      # as a canary instead of testing all build types.
+      - name: Test Build
+        run: nix develop -c zig build -Dapp-runtime=gtk -Dgtk-adwaita=true
+
       - name: Create pull request
         uses: peter-evans/create-pull-request@v7
         with: