Clean image folder and archive file before downloading/generating those
This matters when reusing output folders (as binary factory does for example), as we then retain outdated screenshots and just keep adding files to an already existing fastlane archive.
This fixes e.g. https://binary-factory.kde.org/view/Android/job/Itinerary_android/lastSuccessfulBuild/artifact/fastlane-org.kde.itinerary.zip containing some screenshot files twice.