gitlab CI: update arch linux source image

The old one is deprecated (and removed), new one is archlinux/base which also
now requires the diffutils package.

The new one apparently doesn't come with /var/cache/pacman/pkg and it's
not created, so pacman clean exits with an error, breaking the build.
Simply create that directory and everything is hunky, though dory remains
elusive.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>