win, child_process: sanitize env variables

On Windows environment variables are case-insensitive. When spawning
child process certain apps can get confused if some of the variables are
duplicated.

This adds a step on Windows to normalizeSpawnArguments that removes such
duplicates, keeping only the first (in lexicographic order) entry in the
env key of options. This is partly already done for the PATH entry.

Fixes: https://github.com/nodejs/node/issues/35129

PR-URL: https://github.com/nodejs/node/pull/35210


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>