Reenable macOS .app export from Windows, add warnings about Unix permissions.

[bruvzg]

Fixes #96659

[akien-mga]

Might be worth adding a note about this on https://docs.godotengine.org/en/stable/tutorials/export/exporting_for_macos.html
The first paragraph describes the possible export formats, it could be expanded a bit to make clearer which ones can be exported from which platform, and what to do to make a .app exported from Windows actually usable on macOS.

[bruvzg]

Might be worth adding a note about this

godotengine/godot-docs#9921

[akien-mga]

Thanks!