src: pass isMainThread into bootstrap/node.js directly

Instead of loading the working binding for the sole purpose of
detecting whether we are inside the main thread unconditionally.

PR-URL: https://github.com/nodejs/node/pull/25017


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>