policy: ensure workers do not read fs for policy

[bmeck]

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

This prevents a main file of:

// find the file
const policyPath = findPath(process.execArgv);

// rewrite with out new escalated privileges
fs.writeFileSync(policyPath, modifiedPolicy);

// spawn worker to get the modified policy
new Worker(...);

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

@bmeck build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/2390/pipeline

[joyeecheung]

CI: https://ci.nodejs.org/job/node-test-pull-request/20372/

[addaleax]

Resume CI: https://ci.nodejs.org/job/node-test-pull-request/20390/

[danbev]

Landed in 7898238.