Pull nixpkgs as tarball from channels.nixos.org

Actually working solution for https://github.com/NixOS/flake-registry/pull/61.
Don't even ask what "exact" is doing here, but it's load-bearing for this to
work. Stuff like `nix flake metadata nixpkgs/nixos-26.05` uses the channel tarballs,
while `nix flake metadata nixpkgs/master` falls back to `github:nixos/nixpkgs/master`.

flake-registry.json

@@ -331,6 +331,89 @@
         "type": "github"
       }
     },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz"
+      }
+    },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect",
+        "ref": "nixpkgs-unstable"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz"
+      }
+    },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect",
+        "ref": "nixos-unstable"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz"
+      }
+    },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect",
+        "ref": "nixos-unstable-small"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixos-unstable-small/nixexprs.tar.xz"
+      }
+    },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect",
+        "ref": "nixos-26.05"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixos-26.05/nixexprs.tar.xz"
+      }
+    },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect",
+        "ref": "nixos-26.05-small"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixos-26.05-small/nixexprs.tar.xz"
+      }
+    },
+    {
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect",
+        "ref": "nixpkgs-26.05-darwin"
+      },
+      "exact": true,
+      "to": {
+        "type": "tarball",
+        "url": "https://channels.nixos.org/nixpkgs-26.05-darwin/nixexprs.tar.xz"
+      }
+    },
     {
       "from": {
         "id": "nixpkgs",