Worker: remove secret from env

[josephjclark]

The worker start script, once the server has started, should delete WORKER_SECRET from the env.

This ensures that once the worker instance has the secret in memory, any attacker on the server can no longer access it.

This also means that the server can't be restarted without the container being destroyed (which is probably good)

[josephjclark]

Obviously you can't just do process.env, you have to properly unset it