Set repodata mtime to SOURCE_DATE_EPOCH
    
    repodata/repomd.xml include timestamps of all the other repodata files.
    Even when those files are created reproducibly, they have current
    modification time. In general case this is a good thing (ease checking
    if repodata cache is up to date). But in case of composing installation
    image, it breaks reproducibility.
    Avoid this by instructing createrepo to set mtime and revision to
    $SOURCE_DATE_EPOCH.
    
    Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>