Fix inverting the user cache
    
    In the previous commit we changed the key from ``bugzilla_email``
    to simply ``email`` to be consistent with the data we retrieve
    from FAS, but it looks like we forgot to update this method which
    thus results in the script failing with a KeyError on
    ``bugzilla_email``.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>