[KUiServerJobTracker] Update destUrl before finishing the job

In case a job prompts the user for a location after it has started, destUrl might change or
only be known after the job has already been registered to the job tracker.

While this patch doesn't monitor the property live, it ensures that it is correct at least
when the job has finished, to allow for potentially better feedback about a job having finished.

Differential Revision: https://phabricator.kde.org/D19583