GH-94182: Run the PidfdChildWatcher on the running loop (#94184)

There is no reason for this watcher to be attached to any particular loop.
This should make it safe to use regardless of the lifetime of the event loop running in the main thread
(relative to other loops).

Co-authored-by: Yury Selivanov <yury@edgedb.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>