.gitlab/ci/build-host.sh:
Set shebang to /usr/bin/env bash to be more portable/flexible.
Turn all posix statements ([]) to bash style statements ([[]]), as we are using bash.
Terminate the list of parameters to rm or cp with --.

Replace the implementation of finding a local ISO to use with one that relies on a sorted list of potential images.

Use virtio-net-pci for networking with qemu.
Set the cow_spacesize to 4G for the archiso environment.

Use --needed in the call to pacman to not re-install already up-to-date targets.
Attempt a full system upgrade (but ignore the kernel).
Increase the timeout for when installing packages to the archiso environment using pacman to 120s, as a system upgrade
is being done as well.

Use systemctl poweroff -i to shut down the virtual machine as it is more future proof and robust.

.gitlab/ci/build-host.sh:
Add script to be run in a container with access to qemu.
It is a slight modification of arch-boxes' build-host.sh script to cater to the specific archiso requirements.

.gitlab/ci/build-inside-vm.sh:
Add script to be run in virtualized environment, established by build-host.sh.
This script builds the actual archiso profiles and creates checksum for the resulting image files.