Prompt loop when a user backs-out of update installation

[peterlewis]

Currently, if a user backs-out of installing updates, after pressing continue, they will be faced with a looping prompt which will burn through the remaining deferrals.

Steps to reproduce:

1. Be presented with update prompt
2. Select Continue
3. Allow System Preferences to open
4. Have a sudden change of heart and close System Preferences
5. Observe an immediate further prompt to install updates, taking the number of remaining deferrals down by one
6. Rinse and repeat.