[BUGFIX] EM: Scheduler task does not persist to database

The scheduler task for updating extension list of the extension
manager does not persist his data to database. As Scheduler is no
extbase extension and the task is no extbase task this peristence
isn't called at end of script.

Change-Id: I75629874f698f45f1b7dab47636857b5865e8d24
Resolves: #49528
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/21792
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring