fs: change var to let

PR-URL: #30407
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/fs/streams.js

@@ -360,7 +360,7 @@ WriteStream.prototype._writev = function(data, cb) {
   const chunks = new Array(len);
   let size = 0;
 
-  for (var i = 0; i < len; i++) {
+  for (let i = 0; i < len; i++) {
     const chunk = data[i].chunk;
 
     chunks[i] = chunk;