ostree-installer: Copy files without owner
    
    The files created in koji runroot will be owned by root. If the compose
    is done under different user, there could be a problem with copying the
    files preserving the owner. Let's just copy them without that.
    
    Fixes: https://pagure.io/pungi/issue/932
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>