Make nixpkgs that is used for machine configurable

This allows a machine to run off a different nixpkgs branch (e.g.
unstable).

flake.nix

@@ -159,6 +159,7 @@
                    , extraModules ? [ ]
                    , targetHost ? hostname
                    , unlockOverV4 ? true
+                   , nixpkgs ? inputs.nixpkgs
                    }: nixpkgs.lib.nixosSystem rec {
           inherit system;