ostree_build: tee and timestamp the `rpm-ostree` output
    
    Instead of just redirecting it to a log file, let's tee it, so
    simple errors can be read off a screenshot without bothering to
    download the file. Also, let's timestamp it (via `ts` from
    moreutils) so we can see which bits of it take a long time...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>