buildvm: move the s390x varnish cache to buildvm-s390x-24
    
    Right now the varnish cache machine that all the s390x builders use is
    07, but thats a Z/VM instance and seems pretty prone to having slow IO
    or other issues. Moving it to 24 (which is a kvm instance) hopefully
    will improve performance and avoid build failures.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>