benchmark: replace var with let/const in cluster benchmark

PR-URL: https://github.com/nodejs/node/pull/31042
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>

benchmark/cluster/echo.js

@@ -19,10 +19,10 @@ if (cluster.isMaster) {
     serialization
   }) {
     const expectedPerBroadcast = sendsPerBroadcast * workers;
-    var readies = 0;
-    var broadcasts = 0;
-    var msgCount = 0;
-    var data;
+    let readies = 0;
+    let broadcasts = 0;
+    let msgCount = 0;
+    let data;
 
     cluster.settings.serialization = serialization;
 
@@ -37,7 +37,7 @@ if (cluster.isMaster) {
         throw new Error('Unsupported payload type');
     }
 
-    for (var i = 0; i < workers; ++i)
+    for (let i = 0; i < workers; ++i)
       cluster.fork().on('online', onOnline).on('message', onMessage);
 
     function onOnline() {
@@ -48,16 +48,15 @@ if (cluster.isMaster) {
     }
 
     function broadcast() {
-      var id;
       if (broadcasts++ === n) {
         bench.end(n);
-        for (id in cluster.workers)
+        for (const id in cluster.workers)
           cluster.workers[id].disconnect();
         return;
       }
-      for (id in cluster.workers) {
+      for (const id in cluster.workers) {
         const worker = cluster.workers[id];
-        for (var i = 0; i < sendsPerBroadcast; ++i)
+        for (let i = 0; i < sendsPerBroadcast; ++i)
           worker.send(data);
       }
     }