Pull the Arch image from GitHub's registry

A unit test run stopped before it began: Docker Hub refused to hand out
archlinux:latest, "You have reached your unauthenticated pull rate
limit". GitHub's runners share addresses, so the limit is not ours to
stay under.

Arch publishes the same image as ghcr.io/archlinux/archlinux, which has
no such limit from a runner. The three workflow containers and the ISO
build's own container pull it from there.

.github/workflows/install-test.yml

@@ -50,7 +50,7 @@ jobs:
           sudo chmod 0666 /dev/kvm
           docker run --rm --device /dev/kvm -v "$PWD:/w" -w /w \
             -e OMARCHY_INTEGRATION_DISK_CACHE=none \
-            archlinux:latest bash -euo pipefail -c '
+            ghcr.io/archlinux/archlinux:latest bash -euo pipefail -c '
               # pacman 7 fails every sync on a kernel without Landlock, which
               # these runners have; builder/build-iso.sh does the same.
               if ! grep -qw landlock /sys/kernel/security/lsm 2>/dev/null; then