nixos: Make 'nesting.clone' work in NixOS tests

Because nesting.clone calls 'eval-config.nix' manually,
without the 'extraArgs' argument that provides the 'nodes'
argument to nixos modules in nixos tests, evaluating
of 'nesting.clone' definitions would fail with the following error

while evaluating the module argument `nodes' in "<redacted>"
while evaluating the attribute '_module.args.nodes' at undefined position:
attribute 'nodes' missing, at <redacted./nixpkgs/lib/modules.nix:163:28

by not using 'extraArgs' but a nixos module instead, the nodes parameter
gets propagated to the 'eval-config.nix' call that  'nesting.clone'
makes too -  getting rid of the error.

See  https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/activation/top-level.nix#L13-L23
See  https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/build-vms.nix#L27
See  https://github.com/NixOS/nixpkgs/issues/20886#issuecomment-495952149