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/unit-test.yml

@@ -33,7 +33,7 @@ jobs:
         run: |
           docker run -d --name unit --privileged --cgroupns=host \
             -v /sys/fs/cgroup:/sys/fs/cgroup:rw --tmpfs /run --tmpfs /tmp:exec \
-            -v "$PWD:/src:ro" archlinux:latest bash -c '
+            -v "$PWD:/src:ro" ghcr.io/archlinux/archlinux:latest bash -c '
               systemctl mask getty.target console-getty.service \
                 systemd-udev-trigger.service systemd-udevd.service \
                 systemd-vconsole-setup.service