Add the installation of latest archlinux-keyring to CI

.gitlab-ci.yml:
Install the latest archlinux-keyring before installing anything else.
This is to make sure to not run into outdated keys upon updating, which
is a problem because we can currently not even ensure a keyring that is
valid longer than one month...
archlinux-keyring#4
7 jobs for install_archlinux_keyring in 39 minutes and 54 seconds (queued for 6 seconds)