Switch to arch-boxes for the CI build environment
Using the monthly ISO in QEMU as the CI build environment is becoming more and more unfeasible.
At the end of the month, the live environment needs a lot of package upgrades which take up time and memory (since the rootfs is a overlayfs using tmpfs for upperdir). This causes an out of memory situation.
We should consider using archlinux/arch-boxes> for the CI build environment. The images are updated twice a month and the rootfs is not backed by RAM.