Fix check_expired_networkstatus_callback() if condition

The condition was always true meaning that we would reconsider updating our
directory information every 2 minutes.

If valid_until is 6am today, then now - 24h == 1pm yesterday which means that
"valid_until < (now - 24h)" is false. But at 6:01am tomorrow, "valid_until <
(now - 24h)" becomes true which is that point that we shouldn't trust the
consensus anymore.

Fixes #23091

Signed-off-by: David Goulet <dgoulet@torproject.org>