Fix iOS exports never embedding framework bundles

[mihe]

This partially reverts #86288.

Currently when exporting projects for iOS that (for example) rely on a GDExtension which is built as a .framework bundle, you will find that while said .framework bundle is exported along with the Xcode project, it doesn't actually get embedded into the app bundle when building/archiving, which breaks the application.

This seems to be because the iOS export plugin currently treats .framework the same as .xcframework, where it expects there to be an AvailableLibraries key in the bundle Info.plist, which doesn't seem to be a thing for regular .framework bundles.

This PR fixes this issue by going back to the behavior prior to #86288 for .framework bundles specifically, where IOSExportAsset::should_embed is always respected.

[akien-mga]

Thanks!