lib: change var to let in lib/_stream_duplex.js

PR-URL: #30297
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/_stream_duplex.js

@@ -39,7 +39,7 @@ Object.setPrototypeOf(Duplex, Readable);
 {
   // Allow the keys array to be GC'ed.
   const keys = Object.keys(Writable.prototype);
-  for (var v = 0; v < keys.length; v++) {
+  for (let v = 0; v < keys.length; v++) {
     const method = keys[v];
     if (!Duplex.prototype[method])
       Duplex.prototype[method] = Writable.prototype[method];