If a signing identity is given, create a signed install, and zip it up
(if zip files are wanted).
Can be used to directly add a signed app to an installer.
Change-Id: Ib575d7c907eea74c247407107884f29a526d5d1c
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>