pause: fix trigger whilst paused logic

* Closes #7016
* The wrong indicator was being used to list tasks for job preparation
  resulting in duplicate preparation calls.

Author: oliver-sanders

cylc/flow/scheduler.py

@@ -1354,7 +1354,7 @@ def release_tasks_to_run(self) -> bool:
                 # finish processing preparing tasks
                 pre_prep_tasks.update({
                     itask for itask in self.pool.get_tasks()
-                    if itask.state(TASK_STATUS_PREPARING)
+                    if itask.waiting_on_job_prep
                 })
             else:
                 # release queued tasks