core/path: do not install new job in .trigger_notify() - #27451

Otherwise,
1. X.path triggered X.service, and the service has waiting start job,
2. systemctl stop X.service
3. the waiting start job is cancelled to install new stop job,
4. path_trigger_notify() is called, and may reinstall new start job,
5. the stop job cannot be installed, and triggeres assertion.

So, instead, let's add a defer event source, then enqueue the new start
job after the stop (or any other type) job finished.

Fixes systemd#24577 (comment).