Replace packer with two custom shell scripts

Using actual VMs to build VMs is slow and error-prone (you need to use
VNC to see what is going on, and booting takes over +110 seconds as we
wait to be sure Arch Linux is ready).

build.sh can build all three images in ~135 seconds (assuming all the
packages is cached), we still need to use a VM for the actually building
in GitLab CI (as that is the only safe way it can be done at the
moment), which is a bit slower (~22 min vs ~13 min (Packer)), but that
isn't really a big issue.

In the future we can hopefully switch to Kate Containers[1] and reduce
the build time significantly.

[1] https://gitlab.archlinux.org/archlinux/infrastructure/-/issues/108

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It is getting late, so I haven't got time to test the whole script (create_image isn't tested). It lacks the vagrant images and the box creating logic (probably +100 lines with some inlining).

/cc @svenstaro @shibumi