nixos/virtualbox: fix systemd-networkd-wait-online.service waiting for vboxnet0

While switching NixOS configurations with both

networking.useNetworkd = true;
virtualisation.virtualbox.host.enable;

You often end up waiting for systemd-networkd-wait-online.service.

This happens because the vboxnet0 device doesn't have a carrier until
virtualbox machines are started, so networkd gets stuck in
"Configuring":

⇒  networkctl list
IDX LINK          TYPE      OPERATIONAL SETUP
  1 lo            loopback  carrier     unmanaged
  2 wlp2s0        wlan      routable    unmanaged
  3 vboxnet0      ether     no-carrier  configuring

This updates the NixOS virtualbox host module to include a
RequiredForOnline=no statement in the generated 40-vboxnet0.network
file, so networkd doesn't consider it necessary for
systemd-networkd-wait-online.service to finish.

nixos/modules/virtualisation/virtualbox-host.nix

@@ -149,5 +149,12 @@ in
     # Make sure NetworkManager won't assume this interface being up
     # means we have internet access.
     networking.networkmanager.unmanaged = ["vboxnet0"];
-  })]);
+  }) (mkIf config.networking.useNetworkd {
+    systemd.network.networks."40-vboxnet0".extraConfig = ''
+      [Link]
+      RequiredForOnline=no
+    '';
+  })
+
+]);
 }