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  NixOS#20886 (comment)

nixos/lib/build-vms.nix

@@ -37,9 +37,9 @@ rec {
           ../modules/testing/test-instrumentation.nix # !!! should only get added for automated test runs
           { key = "no-manual"; documentation.nixos.enable = false; }
           { key = "qemu"; system.build.qemu = qemu; }
+          { key = "nodes"; _module.args.nodes = nodes; }
         ] ++ optional minimal ../modules/testing/minimal-kernel.nix
           ++ extraConfigurations;
-      extraArgs = { inherit nodes; };
     };