benchmark: use let instead of var in dgram

PR-URL: https://github.com/nodejs/node/pull/31175


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>