gitlab_runner: Initial the keyring in the base image for faster boot

"Disabling revoked keys in keyring" when running "pacman-key --populate"
is very slow (easily +20 seconds), in our case the boot is now ~27
seconds faster (tested on secure-runner1). The pacman master private key
is removed to prevent malicious actors from injecting packages, a new
key is generated by pacman-init.service on boot.