build-export: Use the --sandbox option for icon validation
build-export: Use the --sandbox option for icon validation

Instead of setting up the sandbox ourselves, rely
on the --sandbox option to do so.

Closes: #2618
Approved by: alexlarsson

flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak