networking: fix in-flight wifi (dns)

Fixes issue on in-flight wifi where I cannot access the login page.

Turns out, `insertNameservers` goes before the delta wifi nameserver,
this prevents `wifi.delta.com` from being resolved. So I need to use
`appendNameservers` instead.

system/networking.nix

@@ -9,7 +9,7 @@
     networkmanager = {
       enable = true;
 
-      insertNameservers = [
+      appendNameservers = [
         "1.1.1.1"
         "9.9.9.9"
       ];