nixos/tests: fix userborn-immutable-etc test

/etc/resolvconf.conf tries to modify files in /etc which obviously
doesn't work with an immutable /etc. Use systemd-networkd instead which
doesn't touch files in /etc.

nixos/tests/userborn-immutable-etc.nix

···
   common = {
     services.userborn.enable = true;
     boot.initrd.systemd.enable = true;
+    networking.useNetworkd = true;
     system.etc.overlay = {
       enable = true;
       mutable = false;