set override_of on new decision

[eviljeff]

Fixes: mozilla/addons#16171

Description

fixes regression from #24679 - we stopped setting override_of on new decisions, where the job already had a decision.

Context

Testing

• create a decision on a job in Cinder and replay the payload
• override that decision in Cinder and replay the payload
• the correct email should be sent (that refers to an earlier decision

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.