nixos/ups: run killpower driver as root

upsdrv starts the driver with `upsdrvctl -u root start`, but
ups-killpower invokes a bare `upsdrvctl shutdown`. Since nut
is built --with-user=nutmon, nut fails to read both /var/lib/nut
and the USB device with "insufficient permissions on everything".
The UPS is then never told to cut its outlets: it keeps draining
battery after the OS has halted, and cannot power-cycle the machine
when mains returns.

Ergo, run the shutdown as root.

(cherry picked from commit f4d444ddff)

nixos/modules/services/monitoring/ups.nix

@@ -679,7 +679,7 @@ in
       environment = envVars;
       serviceConfig = {
         Type = "oneshot";
-        ExecStart = "${cfg.package}/bin/upsdrvctl shutdown";
+        ExecStart = "${cfg.package}/bin/upsdrvctl -u root shutdown";
       };
     };