maint: Rename autobuild.sh to prepare-release.sh
    
    The script was originally used by the Test::AutoBuild
    project to perform periodic automatic builds; however, that
    effort has been abandoned a long time ago, and these days
    virt-viewer CI builds are happening on the Jenkins-based
    CentOS CI environment under the libvirt umbrella[1], where
    build recipes are maintained separately from the projects
    themselves.
    
    The script is still used to prepare releases, so it can't
    be dropped from the repository: rename it so that its
    purpose is more clearly communicated instead.
    
    [1] https://ci.centos.org/view/libvirt/
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>