fix missing extension. Closes #4400

scripts/start-env.js

@@ -1,5 +1,5 @@
 import 'dotenv/config';
-import cli from 'next/dist/cli/next-start';
+import cli from 'next/dist/cli/next-start.js';
 
 cli.nextStart({
   port: process.env.PORT || 3000,