flake: add extraSpecialArgs

This allows flake user to pass extra arguments to all modules.

flake.nix

@@ -9,11 +9,11 @@
     lib = {
       hm = import ./modules/lib { lib = nixpkgs.lib; };
       homeManagerConfiguration = { configuration, system, homeDirectory
-        , username
+        , username, extraSpecialArgs ? { }
         , pkgs ? builtins.getAttr system nixpkgs.outputs.legacyPackages
         , check ? true }@args:
         import ./modules {
-          inherit pkgs check;
+          inherit pkgs check extraSpecialArgs;
           configuration = { ... }: {
             imports = [ configuration ];
             home = { inherit homeDirectory username; };

modules/default.nix

@@ -4,6 +4,8 @@
 
   # Whether to check that each option has a matching declaration.
 , check ? true
+  # Extra arguments passed to specialArgs.
+, extraSpecialArgs ? { }
 }:
 
 with lib;
@@ -31,7 +33,7 @@ let
     modules = [ configuration ] ++ hmModules;
     specialArgs = {
       modulesPath = builtins.toString ./.;
-    };
+    } // extraSpecialArgs;
   };
 
   module = showWarnings (