worker: perform initial port.unref() before preload modules

The refcount of the internal communication port is relevant for
stdio, but the `port.unref()` call effectively resets any `.ref()`
calls happening during stdio operations happening before it.

Therefore, do the `.unref()` call before loading preload modules,
which may cause stdio operations.

Fixes: https://github.com/nodejs/node/issues/31777

PR-URL: https://github.com/nodejs/node/pull/33455


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>