nixosConfigurations: move input forwarding to inline module

Inspired by https://jade.fyi/blog/flakes-arent-real/, this is better for
composability:
>  it also doesn't work outside of the flake that's actually invoking nixpkgs.lib.nixosSystem. The latter is the much more sinister part, and the reason I would strongly recommend inline modules with closures instead of specialArgs: they break flake composition.

For now, the `system` remains a specialArg, until I decide what to do
with that.

common/default.nix

@@ -1,6 +1,7 @@
-{ config, pkgs, lib, inputs, ... }:
+{ config, pkgs, lib, ... }:
 
 let
+  inputs = config.inputInjection.flake-inputs;
   unstable = inputs.nixos-unstable;
 
 in

flake.nix

@@ -31,13 +31,24 @@
     # FIXME: allow different systems
     systems = flake-utils.lib.system;
     # necessary to make the top-level inputs available to system configuration
+    # inspired by https://jade.fyi/blog/flakes-arent-real/
+    inputInjection = {pkgs, lib, ...}: {
+      options.inputInjection = lib.mkOption {
+        type = with lib.types; attrsOf unspecified;
+        default = {};
+      };
+      config.inputInjection = {
+        flake-inputs = inputs;
+      };
+    };
     defaultModules = system: [
-      { _module.args = { inherit inputs system; }; }
+      inputInjection
     ];
     mkSystem = system: extraModules:
       nixpkgs.lib.nixosSystem rec {
         modules = (defaultModules system) ++ extraModules;
         inherit system;
+        specialArgs = {inherit system;};
     };
     mkHomeManager = confName: user: system:  # unfortunately, home-manager configs are still system-specific
     # FIXME: this is thus still linux-x86_64 specific

hosts/framenix/default.nix

@@ -1,6 +1,6 @@
-{ config, lib, pkgs, inputs, system, ...}:
+{ config, lib, pkgs, system, ...}:
 let
-  unstable = inputs.nixos-unstable.legacyPackages.${system};
+  unstable = config.inputInjection.flake-inputs.nixos-unstable.legacyPackages.${system};
 in
 {
   imports = [

hosts/thinknix/default.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, inputs, ...}:
+{ config, lib, pkgs, ...}:
 {
   imports = [
     ../../common

hosts/thinknix/hardware-configuration.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, modulesPath, inputs, ... }:
+{ config, lib, pkgs, modulesPath, ... }:
 
 
 {

hosts/thinknix/secureboot.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, inputs, ...}:
+{ config, lib, pkgs, ...}:
 {  
   boot.loader.efi.canTouchEfiVariables = true;
 

hosts/thinknix/storage.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, inputs, ...}:
+{ config, lib, pkgs, ...}:
 let
   fsOptions = [ "noatime" "ssd" "space_cache" "compress=zstd" ];
 in