benchmark: use let instead of var in dns

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/dns/lookup.js

@@ -10,7 +10,7 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ name, n, all }) {
-  var i = 0;
+  let i = 0;
 
   if (all === 'true') {
     const opts = { all: true };