spam-o-matic: set dnf `Base.conf.arch` to fix reports (#7931)
    
    Thanks to mhroncok for suggesting this. @kevin pointed out that
    reports were still completely wrong on some arches. It seems
    that dnf filtering by provides needs this `Base.conf.arch`
    config option set to the relevant arch to work properly. With
    this set, generating a report for a different arch from the
    'native' arch of the system generating the report should work
    properly.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>