benchmark: use let instead of var in common.js

PR-URL: #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/common.js

@@ -268,7 +268,7 @@ function formatResult(data) {
     conf += ` ${key}=${JSON.stringify(data.conf[key])}`;
   }
 
-  var rate = data.rate.toString().split('.');
+  let rate = data.rate.toString().split('.');
   rate[0] = rate[0].replace(/(\d)(?=(?:\d\d\d)+(?!\d))/g, '$1,');
   rate = (rate[1] ? rate.join('.') : rate[0]);
   return `${data.name}${conf}: ${rate}`;