utils._RunCmdPipe: simplify the timeout logic

There are two issues: first, it's simpler to assign directly to pt
instead of via another local variable. Second, this code doesn't make
sense:
  if lt < 0:
    break
  pt = max(0, lt)

If lt is indeed < 0, then the code will break. Otherwise, lt ≥ 0, and
max(0, lt) will always be equal to lt. So we remove the extra
assignment.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>