nixos/ups: replace activationScript via tmpfiles

nixos/modules/services/monitoring/ups.nix

···
 
     power.ups.schedulerRules = mkDefault "${pkgs.nut}/etc/upssched.conf.sample";
 
-    system.activationScripts.upsSetup = stringAfter [ "users" "groups" ]
-      ''
-        # Used to store pid files of drivers.
-        mkdir -p /var/state/ups
-      '';
+    systemd.tmpfiles.rules = [
+      "d /var/state/ups -"
+    ];
 
 
 /*