benchmark: swap var for let in benchmarks

In benchmark directory this changes for loops
using var to let when it applies for consistency

PR-URL: https://github.com/nodejs/node/pull/28958
Reviewed-By: Anna Henningsen <anna@addaleax.net>

benchmark/string_decoder/string-decoder-create.js

@@ -11,7 +11,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ encoding, n }) {
   bench.start();
-  for (var i = 0; i < n; ++i) {
+  for (let i = 0; i < n; ++i) {
     const sd = new StringDecoder(encoding);
     !!sd.encoding;
   }