benchmark: use let instead of var in worker

PR-URL: https://github.com/nodejs/node/pull/31794
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

benchmark/worker/echo.js

@@ -55,7 +55,7 @@ function main({ n, workers, sendsPerBroadcast: sends, payload: payloadType }) {
       return;
     }
     for (const worker of workerObjs) {
-      for (var i = 0; i < sends; ++i)
+      for (let i = 0; i < sends; ++i)
         worker.postMessage(payload);
     }
   }