process: improve cwd performance

This caches the current working directory and only updates the variable
if `process.chdir()` is called.

PR-URL: https://github.com/nodejs/node/pull/27224


Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>