src: modified RealEnvStore methods to use libuv functions

Modified RealEnvStore::Get, Set, Query and Delete methods
to use libuv methods environment variables operations instead
of using os specific logic and switches.

Fixes: https://github.com/nodejs/node/issues/27211
Refs: http://docs.libuv.org/en/v1.x/misc.html

PR-URL: https://github.com/nodejs/node/pull/27310


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>