flake: switch to nixpkgs via the Tarball protocol

see https://github.com/NixOS/infra/pull/562

flake.lock

@@ -800,18 +800,15 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1751792365,
-        "narHash": "sha256-J1kI6oAj25IG4EdVlg2hQz8NZTBNYvIS0l4wpr9KcUo=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "1fd8bada0b6117e6c7eb54aad5813023eed37ccb",
-        "type": "github"
+        "lastModified": 315532800,
+        "narHash": "sha256-5EzvWAMTAf6m6Ynab7EnYz9OfJO2wHCb/yhtP8ACQQQ=",
+        "rev": "9b008d60392981ad674e04016d25619281550a9d",
+        "type": "tarball",
+        "url": "https://releases.nixos.org/nixpkgs/nixpkgs-25.11pre826760.9b008d603929/nixexprs.tar.xz"
       },
       "original": {
-        "owner": "NixOS",
-        "ref": "nixos-unstable",
-        "repo": "nixpkgs",
-        "type": "github"
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz"
       }
     },
     "nixpkgs_3": {

flake.nix

@@ -40,8 +40,9 @@
     templates = import ./templates;
   };
   inputs = {
-    # Unstable nixpkgs baby!
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    # Unstable nixpkgs, using the new Lockable HTTP Tarball protocol
+    # https://github.com/NixOS/infra/pull/562.
+    nixpkgs.url = "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz";
 
     systems.url = "github:nix-systems/default-linux";