child_process: do not ignore proto values of env

This reverts this behaviour introduced in a recent PR, and updates
the test. Without this change, CitGM and other packages are broken.

PR-URL: https://github.com/nodejs/node/pull/18210
Fixes: https://github.com/nodejs/citgm/issues/536


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>