fix locked DMG after installing Qt OSX package and starting creator

  - the workingdir was inside the dmg so it couldn't unmounted

Task-number: QTBUG-28963

Change-Id: Idac6dc99d8f5070cc877b544b52fd9b14a6bf528
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>