Add script to build in archiso vm
scripts/build_in_archiso_vm.sh: Add script to execute a build script inside an archiso based VM. The script is based on the latest development in releng (https://gitlab.archlinux.org/archlinux/releng/-/blob/f3b03c7dfd12defc99d70e322fd0b571bbddc655/template/build-host.sh) which has initially been taken from arch-boxes (https://gitlab.archlinux.org/archlinux/arch-boxes/-/blob/5ca90361a7d446b498909a901779cdff3a4018ad/build-host.sh), refined for the use in archiso (https://gitlab.archlinux.org/archlinux/archiso/-/blob/960b988ac6dc962c94bb020aac7fe9a85874aa38/.gitlab/ci/build-host.sh). Add SPDX license identifier for GPL-3.0-or-later. Fix script to be compliant with shfmt.
Closes #1 (closed)