[backend] don't rely on createrepo from prunerepo
    
    Fix #646
    
    Prunerepo is able to run createrepo_c command after pruning
    but it has limited possibilities. For example, it cannot create
    appstream data, see #646. Instead of trying keep copr features
    in sync with createrepo_c possibilities in prunerepo, rather
    run prunerepo with `--nocreaterepo` and run our createrepo
    function afterwards.