prompted to disallow tasks when already in settings

[eleanorjboyd]

Testing #290461

1. clean user dir
2. open workspace with auto task
3. disable auto task on open
4. see it in user settings
5. open a different repo with auto tasks
6. 🐛 see the notification about the allowing tasks again even though the setting is updated

setting-disallow-twice.mov

[eleanorjboyd]

the same did not happen if I had "task.allowAutomaticTasks": "true" in my settings, I was not prompted and the task was auto-run

[meganrogge]

This is similar to #290993 and we'll have the same fix. If the user explicitly sets it to off, we won't prompt. If it's missing, thus the default is off, we will prompt once per workspace.