nixf: 2.6.3 -> 2.6.4

pkgs/development/tools/language-servers/nixd/default.nix

···
 }:
 
 let
+  nix = nixVersions.nix_2_28;
   common = rec {
-    version = "2.6.3";
+    version = "2.6.4";
 
     src = fetchFromGitHub {
       owner = "nix-community";
       repo = "nixd";
       tag = version;
-      hash = "sha256-Gd7VFyQ/ayw0NI72sdZ1wFuXaxlIPWyE31Kl53d3zB4=";
+      hash = "sha256-K7S626SPzlNCmRhntSKhGP1iyHJXBZEeHliX4iEwbKk=";
     };
 
     nativeBuildInputs = [
···
       ];
 
       buildInputs = [
-        nixVersions.nix_2_24
+        nix
         gtest
         boost
       ];
-
-      env.CXXFLAGS = "-include ${nixVersions.nix_2_24.dev}/include/nix/config.h";
 
       passthru.tests.pkg-config = testers.hasPkgConfigModules {
         package = nixt;
···
       sourceRoot = "${common.src.name}/nixd";
 
       buildInputs = [
-        nixVersions.nix_2_24
+        nix
         nixf
         nixt
         llvmPackages.llvm
···
       ];
 
       nativeBuildInputs = common.nativeBuildInputs ++ [ cmake ];
-
-      env.CXXFLAGS = "-include ${nixVersions.nix_2_24.dev}/include/nix/config.h";
 
       # See https://github.com/nix-community/nixd/issues/519
       doCheck = false;