policy: ensure workers do not read fs for policy

This prevents a main thread from rewriting the policy file and loading
a worker that has a different policy from the main thread.

PR-URL: https://github.com/nodejs/node/pull/25710


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>