ci: Only build the flatpak runtimes on i686
Avoid build bumping into build failures on those platforms for the rest of the elements as we only need to distribute the i386 Compat extension and don't need to build the rest of the element.
Avoid build bumping into build failures on those platforms for the rest of the elements as we only need to distribute the i386 Compat extension and don't need to build the rest of the element.