🏗: add sctipt for postinstall (#6255)

package.json

@@ -4,6 +4,7 @@
   "private": true,
   "description": "An out-of-box UI solution for enterprise applications",
   "scripts": {
+    "postinstall": "umi g tmp",
     "analyze": "cross-env ANALYZE=1 umi build",
     "build": "umi build",
     "deploy": "npm run site && npm run gh-pages",