fix: nix path (#147)

modules/base.nix

@@ -2,6 +2,8 @@
   pkgs,
   myvars,
   nuenv,
+  nixpkgs,
+  lib,
   ...
 } @ args: {
   nixpkgs.overlays =
@@ -111,4 +113,12 @@
     ];
     builders-use-substitutes = true;
   };
+
+  # make `nix run nixpkgs#nixpkgs` use the same nixpkgs as the one used by this flake.
+  nix.registry.nixpkgs.flake = nixpkgs;
+
+  environment.etc."nix/inputs/nixpkgs".source = "${nixpkgs}";
+  # make `nix repl '<nixpkgs>'` use the same nixpkgs as the one used by this flake.
+  # discard all the default paths, and only use the one from this flake.
+  nix.nixPath = lib.mkForce ["/etc/nix/inputs"];
 }