chore: Update llama.cpp dependency to b4681

- Update flake.lock and flake.nix to use llama.cpp rev d7b31a9d84297b493a61c9a8ee3a458e7ccc64a7
- Re-enable nixcache.vlt81.de substituter and trusted public key

flake.lock

@@ -64,16 +64,16 @@
         ]
       },
       "locked": {
-        "lastModified": 1738311869,
-        "narHash": "sha256-eHLw3tO+t9fP/9+EV4VpfLv+PJo7poSpow0yMvycm+U=",
+        "lastModified": 1739180049,
+        "narHash": "sha256-LjIIeMqGJVv6rHsnf5tWhejiysD3LIsOplwE+M3YeZg=",
         "owner": "ggerganov",
         "repo": "llama.cpp",
-        "rev": "5783575c9d99c4d9370495800663aa5397ceb0be",
+        "rev": "d7b31a9d84297b493a61c9a8ee3a458e7ccc64a7",
         "type": "github"
       },
       "original": {
         "owner": "ggerganov",
-        "ref": "b4604",
+        "ref": "b4681",
         "repo": "llama.cpp",
         "type": "github"
       }

flake.nix

@@ -2,12 +2,12 @@
   description = "A Nix-flake-based Rust development environment";
   nixConfig = {
     extra-substituters = [
-      # "https://nixcache.vlt81.de"
+      "https://nixcache.vlt81.de"
       "https://llama-cpp.cachix.org"
       "https://cuda-maintainers.cachix.org"
     ];
     extra-trusted-public-keys = [
-      # "nixcache.vlt81.de:nw0FfUpePtL6P3IMNT9X6oln0Wg9REZINtkkI9SisqQ="
+      "nixcache.vlt81.de:nw0FfUpePtL6P3IMNT9X6oln0Wg9REZINtkkI9SisqQ="
       "llama-cpp.cachix.org-1:H75X+w83wUKTIPSO1KWy9ADUrzThyGs8P5tmAbkWhQc="
       "cuda-maintainers.cachix.org-1:0dq3bujKpuEPMCX6U4WylrUDZ9JyUG0VpVZa7CNfq5E="
     ];
@@ -23,7 +23,7 @@
       flake = false;
     };
     llama-cpp = {
-      url = "github:ggerganov/llama.cpp/b4604";
+      url = "github:ggerganov/llama.cpp/b4681";
       inputs.nixpkgs.follows = "nixpkgs";
       inputs.flake-parts.follows = "flake-parts";
     };