nixos/networking-interfaces-scripted: add fix for systemd initrd

When systemd is used in stage 1 (boot.initrd.systemd.enable), the device
units are activated before the stage 2 units are loaded, so the
network-addresses-*.service are never started.

To avoid this, in addition to the wantedBy dependency, we use udev rules
with SYSTEMD_WANTS to trigger the services directly.

nixos/modules/tasks/network-interfaces-scripted.nix

@@ -888,9 +888,18 @@ let
       description = "NixOS scripted networking setup";
     };
 
-    services.udev.extraRules = ''
-      KERNEL=="tun", TAG+="systemd"
-    '';
+    services.udev.extraRules = lib.concatStringsSep "\n" (
+      [ ''KERNEL=="tun", TAG+="systemd"'' ]
+      # This creates a udev rule to start each service with a WantedBy
+      # dependency on a device unit. It's needed because if the service
+      # unit is loaded in stage 2 but its device was already up by
+      # stage 1, systemd will not automatically start it.
+      ++ lib.forEach (lib.attrNames cfg.interfaces) (
+        iface:
+        ''ACTION=="add", SUBSYSTEM=="net", KERNEL=="${iface}", ''
+        + ''ENV{SYSTEMD_WANTS}="network-addresses-${iface}.service"''
+      )
+    );
 
   };