backend: better call_copr_repo logging
    
    Log the `copr-repo` PID automatically using the `run_cmd` helper.  It
    was otherwise very complicated to pair the `modifyrepo.log` with
    `pruner.log` (and others).
    
    While on it, let's always execute call_copr_repo with the logger
    argument, this will make the log output (on failures especially) much
    more useful, and everywhere.  The note about `modifyrepo.log` applies
    here, too.