ostree_installer: Pass --buildarch to lorax
    
    This should tell lorax what arch to use and avoids fragile detection
    based on contents of source repo.
    
    It uses the same logic buildinstall phase uses to get the buildarch.
    
    Related: https://pagure.io/teamsilverblue/issue/67
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>