common: Set ~/.local/share/xdg-app/system-cache as cache dir for system repo
common: Set ~/.local/share/xdg-app/system-cache as cache dir for system repo

This means we can use a summary cache for system wide stuff which causes
loads to be faster.

handsome-feng/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,alexlarsson/xdg-app,alexlarsson/xdg-app,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,amigadave/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,alexlarsson/xdg-app,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,alexlarsson/xdg-app,matthiasclasen/flatpak,matthiasclasen/flatpak,amigadave/flatpak,flatpak/flatpak,amigadave/flatpak