src: add NODE_NO_WARNINGS to --help output

This commit adds a description of the NODE_NO_WARNINGS
environment variable to the --help output. This was overlooked
in 49902124.

Refs: https://github.com/nodejs/node/pull/10842
PR-URL: https://github.com/nodejs/node/pull/10918


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Roman Reiss <me@silverwind.io>