Make "nix shell" and "nix-shell" use flake nixpkgs

modules/nix-settings.nix

@@ -1,5 +1,7 @@
-_: {
-  nix.settings = {
+{ pkgs, ... }:
+{
+  nix = {
+    settings = {
       extra-experimental-features = [
         "nix-command"
         "flakes"
@@ -12,4 +14,13 @@ _: {
       # RPi builds can be slow due to compiling via binfmt.
       download-buffer-size = 1 * 1024 * 1024 * 1024;
     };
+
+    # Use nixpkgs of the current flake in "nix run", "nix shell" etc.
+    registry.nixpkgs.to = {
+      type = "path";
+      inherit (pkgs) path;
+    };
+    # Use nixpkgs of the current flake in nix-shell.
+    nixPath = [ "nixpkgs=${pkgs.path}" ];
+  };
 }