path: replace var with let in lib/path.js

PR-URL: https://github.com/nodejs/node/pull/30260
Refs: https://github.com/nodejs/code-and-learn/issues/97


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>