nixos.acme: make timer persistent

This makes sure that if the system was powered off when the timer was
supposed to trigger, it will run the next time the system boots up.

nixos/modules/security/acme.nix

@@ -282,6 +282,7 @@ in
           timerConfig = {
             OnCalendar = cfg.renewInterval;
             Unit = "acme-${cert}.service";
+            Persistent = "yes";
           };
         })
       );