Use snap:// URLs rather than appstream:// ones on snap only distros
Ubuntu Core systems only provide snap as a way to install software so add a way to spot those. To that effect we use the non-standard UBUNTU_VARIANT key in os-release to detect Ubuntu Core.
Not all the snaps in the snapstore declare proper appdata for their package. So using only appstream:// URLs might lead to not found packages on snap only distros like Ubuntu Core. If we detect Ubuntu Core we use snap:// URLs instead.