nixos/manual: add hint to interactive testing about Internet access

verified with following minimal test, embedded in a flake for ease of
execution:
```nix
{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/master";
  outputs = { nixpkgs, ... }:
    let
      pkgs = import nixpkgs { system = "x86_64-linux"; };
    in
    {
      nixosTests.x86_64-linux.internet = pkgs.nixosTest {
        name = "internet";
        nodes.machine = { };
        # only succeeds in interactive mode
        testScript = ''
          machine.wait_for_unit("default.target")
          print(machine.succeed("curl -4 https://nixos.org"))
        '';
      };
    };
}
```

Launch this test interactively as described in the manual & run
`test_script()` to verify that the VM has Internet access.

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