export: Fix IMPORTED_LOCATION of iOS flat app bundles

iOS/tvOS app bundles are flat and do not include the `Contents/MacOS/`
layers, so they should not be in the `IMPORTED_LOCATION`.  Use the same
logic that commit 34f5ef564a (iOS: Fix App Bundle layout, 2015-12-10,
v3.5.0-rc1~129^2~1) added for the installation layout.

Fixes: #23963