lxc-ci: Pacman keyring stored in archlinux image with a private key

lxc-ci contains continuous integration and image-build scripts for LXC. Prior to the 2026-05-28 Arch Linux image publication, images built from images/archlinux.yaml retain the same pacman local-signing private key in /etc/pacman.d/gnupg and redistribute it to every container or virtual machine created from that image. An attacker who controls an HTTP package mirror or can intercept mirror traffic can use the shared pacman signing private key to sign modified packages that affected clients accept as trusted. Installing those packages permits arbitrary code execution as root on the client system. This issue is fixed in Arch Linux images published on or after 2026-05-28.