process: mark process.env as side-effect-free

Read-only access to `process.env` does not have side effects.

Refs: https://github.com/nodejs/node/pull/27523

PR-URL: https://github.com/nodejs/node/pull/27684


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Aleksei Koziatinskii <ak239spb@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>