only build @headlessui/react and @headlessui/vue

This used to also build the individual playground packages but that's
not needed on CI (nor locally). Because Vercel will build them for us.
This will safe us some time on CI, since we can run them in parallel now
and only build the actual libraries.

package.json

@@ -17,7 +17,7 @@
     "playground-vue": "yarn workspace playground-vue dev",
     "vue-playground": "yarn workspace playground-vue dev",
     "clean": "yarn workspaces run clean",
-    "build": "yarn workspaces run build",
+    "build": "npm-run-all -p 'react build' 'vue build'",
     "test": "./scripts/test.sh",
     "lint": "./scripts/lint.sh",
     "lint-check": "CI=true ./scripts/lint.sh"