nixos/dhcpcd: (try to) restart chrony in the exitHook

As the comment notes, restarts/exits of dhcpcd generally require
restarting the NTP service since, if name resolution fails for a pool of
servers, the service might break itself. To be on the safe side, try
restarting Chrony in these instances, too.

Signed-off-by: Austin Seipp <aseipp@pobox.com>