kubernetes vm test: make dig and netcat available

Dig could not be found in the test cases. Adding it as a global package
fixes this.

nixos/tests/kubernetes.nix

···
             virtualisation.diskSize = 2048;
 
             programs.bash.enableCompletion = true;
+            environment.systemPackages = with pkgs; [ netcat bind ];
 
             services.kubernetes.roles = ["master" "node"];
             virtualisation.docker.extraOptions = "--iptables=false --ip-masq=false -b cbr0";