cluster: replace var with let

replace var with let in lib/internal/cluster/utils.js
PR-URL: #30425
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
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/cluster/utils.js

@@ -6,7 +6,7 @@ module.exports = {
 };
 
 const callbacks = new Map();
-var seq = 0;
+let seq = 0;
 
 function sendHelper(proc, message, handle, cb) {
   if (!proc.connected)
@@ -29,7 +29,7 @@ function internal(worker, cb) {
     if (message.cmd !== 'NODE_CLUSTER')
       return;
 
-    var fn = cb;
+    let fn = cb;
 
     if (message.ack !== undefined) {
       const callback = callbacks.get(message.ack);