vulkan-headers: lock rev

pkgs/development/libraries/vulkan-headers/default.nix

···
   src = fetchFromGitHub {
     owner = "KhronosGroup";
     repo = "Vulkan-Headers";
-    rev = "sdk-${version}";
+    rev = "2fd5a24ec4a6df303b2155b3f85b6b8c1d56f6c0";
     sha256 = "0cj4bd396qddh3nxvr7grnpfz89g3sbvm21cx4k3ga52sp1rslpb";
   };