wireguard module: add device name environment var

This makes the interface name available as an environment variable for
the pre/post hooks.

nixos/modules/services/networking/wireguard.nix

@@ -195,6 +195,7 @@ let
         description = "WireGuard Tunnel - ${name}";
         after = [ "network.target" ];
         wantedBy = [ "multi-user.target" ];
+        environment.DEVICE = name;
 
         serviceConfig = {
           Type = "oneshot";