builder: Keep cache per arch
builder: Keep cache per arch

This just adds the arch name to the cache branch names, which
means a build on a different arch will not invalidate the other
arch:s build.

Fixes https://github.com/flatpak/flatpak/issues/628

GeorgesStavracas/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak