Change cmake version

Author: hamphet

shell.nix

@@ -1,13 +1,22 @@
 with import <nixpkgs> {};
 let
+  pkgs = import (builtins.fetchGit {
+    # Descriptive name to make the store path easier to identify
+    name = "cmake24-revision";
+    url = "https://github.com/NixOS/nixpkgs/";
+    ref = "refs/heads/nixpkgs-unstable";
+    rev = "55070e598e0e03d1d116c49b9eff322ef07c6ac6";
+  }) {};
+
+  cmake24 = pkgs.cmake;
   gccForLibs = stdenv.cc.cc;
 in stdenv.mkDerivation {
     name = "llvm-env";
     buildInputs = [
       bashInteractive
       python3
       ninja
-      cmake
+      cmake24
       llvmPackages_latest.llvm
     ];