Pull the Arch image for tests and trackers from GHCR

The self-tests, the build-isolation fixture and the three tracker
workflows pulled archlinux:base-devel from Docker Hub without logging
in. On 2026-10-09 that failed two jobs on one pull request: one with
'toomanyrequests', one with a 504 from Docker Hub's token endpoint.

Arch publishes the same image as ghcr.io/archlinux/archlinux, which
GitHub's runners pull without that limit.

.github/workflows/sync-rebuilds.yml

@@ -68,7 +68,7 @@ jobs:
             -v "$PWD/helpers:/workspace/helpers:ro" \
             -v "$PWD/pkgbuilds:/workspace/pkgbuilds" \
             -w /workspace \
-            archlinux:base-devel bash -lc '
+            ghcr.io/archlinux/archlinux:base-devel bash -lc '
               set -euo pipefail
 
               printf "Server = https://mirror.omarchy.org/\$repo/os/\$arch\n" > /etc/pacman.d/mirrorlist