nixos/flake: put nixpkgs in NIX_PATH and system registry for flake configs

Currently there are a bunch of really wacky hacks required to get nixpkgs
path correctly set up under flake configs such that `nix run
nixpkgs#hello` and `nix run -f '<nixpkgs>' hello` hit the nixpkgs that
the system was built with. In particular you have to use specialArgs or
an anonymous module, and everyone has to include this hack in their
own configs.

We can do this for users automatically.

I have tested these manually with a basic config; I don't know if it is
even possible to write a nixos test for it since you can't really get a
string-with-context to yourself unless you are in a flake context.

flake.nix

@@ -27,7 +27,19 @@
               # We set it to null, to remove the "legacy" entrypoint's
               # non-hermetic default.
               system = null;
-            } // args
+
+              modules = args.modules ++ [
+                # This module is injected here since it exposes the nixpkgs self-path in as
+                # constrained of contexts as possible to avoid more things depending on it and
+                # introducing unnecessary potential fragility to changes in flakes itself.
+                #
+                # See: failed attempt to make pkgs.path not copy when using flakes:
+                # https://github.com/NixOS/nixpkgs/pull/153594#issuecomment-1023287913
+                ({ config, pkgs, lib, ... }: {
+                  config.nixpkgs.flake.source = self.outPath;
+                })
+              ];
+            } // builtins.removeAttrs args [ "modules" ]
           );
       });