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