AttributeError when starting or retrying a process in CE

Sentry Issue: ARKINDEX-BACKEND-2XZ

AttributeError: 'list' object has no attribute 'using'
(10 additional frame(s) were not displayed)
...
  File "contextlib.py", line 80, in inner
    with self._recreate_cm():

In #1960 (closed), we had to make some custom prefetching to manually update the tasks of a process after retrying, so that RetryProcess could return the new tasks without having to read them from the database again and get a stale read. The hacks come with tradeoffs, and schedule_tasks helper used to create RQ tasks to run a process in CE did not handle the fact that process.tasks could have been manually prefetched.