nixos/ups: set env vars in the global environment

This allows using upsdrvctl interactively, which otherwise tries to use
a missing ups.conf in the Nix store, instead of the correct
/etc/nut/ups.conf.

(cherry picked from commit 1cb392fdcd)

nixos/modules/services/monitoring/ups.nix

@@ -501,7 +501,9 @@ in
       })
     ];
 
+    # For interactive use.
     environment.systemPackages = [ pkgs.nut ];
+    environment.variables = envVars;
 
     networking.firewall = mkIf cfg.openFirewall {
       allowedTCPPorts =