build-host.sh: Style and syntax fixes

.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.
4 jobs for issues/2 in 39 minutes and 33 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Lint
passed #14861
shellcheck

00:00:16

 
  Build
passed #14864
fast-single-thread
build: [baseline]

00:21:45

passed #14863
fast-single-thread
build: [releng]

00:39:16

failed #14862
fast-single-thread
build: [baseline]

00:00:48