build: use prepare instead of prepublish

Author: dead-horse

package.json

@@ -17,7 +17,7 @@
     "bench": "make -C benchmarks",
     "authors": "git log --format='%aN <%aE>' | sort -u > AUTHORS",
     "build": "gen-esm-wrapper . ./dist/koa.mjs",
-    "prepublish": "npm run build"
+    "prepare": "npm run build"
   },
   "repository": "koajs/koa",
   "keywords": [