benchmark: use let instead of var

Use `let` in module, napi, net, os, path, process, querystring, streams
and string_decoder.

PR-URL: https://github.com/nodejs/node/pull/31592


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>