nix-darwin,nixos: add osConfig module argument (#2302)

Having either argument defined based on the OS is a problem when
trying to write generic Nix code.

The current workaround is to use accept both and specify a default
value for each argument:

```
{ config, lib, nixosConfig ? {}, darwinConfig ? {}, ... }:

let
  osConfig = nixosConfig // darwinConfig;
in
{
  # Do something with `osConfig`
}
```

With this commit, it becomes possible to do the following:

```
{ config, lib, osConfig, ... }:

{
  # Do something with `osConfig`
}
```

nixos/default.nix

@@ -12,6 +12,7 @@ let
     specialArgs = {
       lib = extendedLib;
       nixosConfig = config;
+      osConfig = config;
       modulesPath = ../modules;
     } // cfg.extraSpecialArgs;
     modules = [