worker: use copy of process.env

Instead of sharing the OS-backed store for all `process.env` instances,
create a copy of `process.env` for every worker that is created.

The copies do not interact. Native-addons do not see modifications to
`process.env` from Worker threads, but child processes started from
Workers do default to the Worker’s copy of `process.env`.

This makes Workers behave like child processes as far as `process.env`
is concerned, and an option corresponding to the `child_process`
module’s `env` option is added to the constructor.

Fixes: https://github.com/nodejs/node/issues/24947

PR-URL: https://github.com/nodejs/node/pull/26544


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>