backend: copr_fix_gpg: properly regenerate DNF metadata
    
    We can not just run "createrepo_c --skip-stat" from copr_fix_gpg.py
    as we otherwise do by default because in copr_fix_gpg.py we are changing
    the RPM GPG signatures (and thus the overall package checksum) – while
    the RPM filename is not changed.  With The "--skip-stat" option then,
    createrepo_c is not able to detect this file change and thus the repo
    data stay untouched (and outdated).
    
    Fixes: #2106
    Merges: #2119