build-export is actually a low-level tool used by flatpak-builder. When
using it directly, debug and locale extensions were not extracted as
separate extensions (unless tweaking complicated command lines), ending
up with a huge GIMP flatpak with the current procedure.
Since flatpak 0.9.5, the option --export-only has been added to
`flatpak-builder` so that the build and the export can be made in 2
separate steps while using the high level procedure.