Move `pacman-key --init` + `--populate` ahead of the first `pacman -Syu`
call in `build_and_install_arch`. The previous guard
`pacman-key --list-sigs` always returned 0 on a fresh Manjaro/Arch image,
so the keyring init never ran and the upgrade of `archlinux-keyring`
failed with `==> ERROR: There is no secret key available to sign with.`,
breaking the manjarolinux/base test-environment matrix entry.
Refs: https://github.com/infinito-nexus/core/issues/167
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>