sea: only assert snapshot main function for main threads

[joyeecheung]

Snapshot main functions are only loaded for main threads in single executable applications. Update the check to avoid asserting it in worker threads - this allows worker threads to be spawned in snapshot main functions bundled into a single executable application.

(Also noticed that worker_threads cannot be loaded in the snapshot builder script, which is to be expected currently since we haven't vetted the worker_threads module for snapshot building. I left a FIXME on how to vet the worker internals and make it loadable in the snapshot builder).

Fixes: https://github.com/nodejs/node/issues/56077