SERIES PREMIERE: Another of television’s oldest traditions is given a timely new twist here. CBS, sensitive that too much exposure to the news is as toxic as too much exposure to other human beings right now, presents a nightly version of its flagship news series, 60 Minutes, that arrives at the end of each evening’s prime-time block on CBS, at only one-tenth the length. This new nightly digest, 6 Minutes, takes everything from the daily presidential briefing and