Some users run "tailscale cert" in a cron job to renew their
certificates on disk. The time until the next cron job run may be long
enough for the old cert to expire with our default heristics.
Add a `--min-validity` flag which ensures that the returned cert is
valid for at least the provided duration (unless it's longer than the
cert lifetime set by Let's Encrypt).
Updates #8725
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
e7bf6e716b