get_bz_query: use last_update_string for bug creation too
    
    The comment from @tflink saying this doesn't work, which this
    commit removes, seems to be no longer valid. I tested using a
    query for creation_ts greaterthaneq "2020-11-27" and then making
    it "2020-11-27 16:00 GMT"; the former gave 202 results, the
    latter gave 101. That looks a lot like it works.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>
    Merges https://pagure.io/fedora-qa/blockerbugs/pull-request/162