src: make Environment::is_stopping_worker inline

Fixes a TODO comment.

PR-URL: https://github.com/nodejs/node/pull/21720


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>