nixos/tzupdate: Remove reference to non-existing attribute

Fixes #402016.

Changed files
-1
nixos
modules
services
-1
nixos/modules/services/misc/tzupdate.nix
···
systemd.timers.tzupdate = {
enable = cfg.timer.enable;
-
interval = cfg.timer.interval;
timerConfig = {
OnStartupSec = "30s";
OnCalendar = cfg.timer.interval;