defaults: add nixpkgs-newest to the registry

defaults/nix.nix

@@ -15,6 +15,32 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    nix.registry = {
+      # the newest version of nixpkgs (which makes sense to fetch)
+      #
+      # this is useful because on a flake based system,
+      # "nixpkgs" is rewritten to a static store path
+      # (the nixpkgs input the system uses)
+      #
+      # that's useful to avoid redownloading nixpkgs constantly,
+      # but for packages that you need the newest version of,
+      # it's not suitable.
+      #
+      # so if you wanted the newest version (eg of yt-dlp)
+      # this is a good way: `nix shell nixpkgs-newest#yt-dlp`
+      nixpkgs-newest = {
+        from = {
+          type = "indirect";
+          id = "nixpkgs-newest";
+        };
+        to = {
+          type = "tarball";
+          # https://chaos.social/@hexa/117111428896044773
+          url = "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.zst";
+        };
+      };
+    };
+
     # enable useful nix tools, and the flakes system
     nix.settings.experimental-features = [
       "nix-command"