fix: address feedbacks

Co-authored-by: Trivikram Kamat <[email protected]>

Author: AllanZhengYP

packages/middleware-sdk-sts/package.json

@@ -2,11 +2,10 @@
   "name": "@aws-sdk/middleware-sdk-sts",
   "version": "3.0.0",
   "scripts": {
-    "prepublishOnly": "yarn build:cjs && yarn build:es",
+    "prepublishOnly": "yarn build && downlevel-dts dist/types dist/types/ts3.4",
     "build:cjs": "tsc -p tsconfig.cjs.json",
     "build:es": "tsc -p tsconfig.es.json",
     "build": "yarn build:es && yarn build:cjs",
-    "postbuild": "downlevel-dts dist/types dist/types/ts3.4",
     "test": "jest"
   },
   "main": "./dist/cjs/index.js",