ostree/utils: Drop timestamps from generated repo names
    
    Since we drop these files in a separate workdir each time,
    there's no need to datestamp them.  Doing so is part of the
    cause for invalidating's rpm-ostree input change hashing.
    
    Issue: https://pagure.io/pungi/issue/811
    Signed-off-by: Colin Walters <walters@verbum.org>