build(docs-infra): add missing `build` npm script for `universal` docs example ZIP archive (#36483)

Previously, the `package.json` template used in the ZIP archive of the
`universal` example that we offer for download missed the `build` npm
script.

This commit updates the template for the `universal` docs example to
include the `build` npm script.

NOTE:
The `build` npm script is already included in
`aio/tools/examples/shared/boilerplate/universal/package.json`, but it
was removed by the example zipper.

PR Close #36483

aio/tools/example-zipper/customizer/package-json/universal.json

@@ -1,6 +1,7 @@
 {
   "scripts": [
     { "name": "ng", "command": "ng" },
+    { "name": "build", "command": "ng build" },
     { "name": "start", "command": "ng serve" },
     { "name": "test", "command": "ng test" },
     { "name": "lint", "command": "ng lint" },