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.

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 = [