ci: Run $(CI_PREPARE_SCRIPT) as root

ci: Run $(CI_PREPARE_SCRIPT) as root

In order for the prepare script to be really useful, it needs
to be able to perform privileged operations such as installing
additional packages or setting up custom mount points.

In order to achieve that, we now run the container as root,
run the prepare script with full privilege, and only then
switch to the unprivileged account with sudo.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>