src: ready background workers before bootstrap

Make sure background workers are ready before proceeding with the
bootstrap or post-bootstrap execution of any code that may trigger
`process.exit()`.

Fixes: https://github.com/nodejs/node/issues/23065

PR-URL: https://github.com/nodejs/node/pull/23233


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>