src: add context for TODO comment in env.cc

Add more context around the TODO change required for achieving the
task. When destructuring the isolate and environment_vars from the
environment object, it is leading to recursive dependency and thus not
able to refactor it in a better way.

PR-URL: https://github.com/nodejs/node/pull/37140


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>