Merge pull request #217958 from NixOS/revert-217851-fix-eval-kiwisolver

pkgs/development/python-modules/kiwisolver/default.nix

···
     hash = "sha256-1BmXUZ/LpKHkbrSi/jG8EvD/lXsrgbrCjbJHRPMz6VU=";
   };
 
-  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin "-I${lib.getDev libcxx}/include/c++/v1";
+  NIX_CFLAGS_COMPILE = lib.optionals stdenv.isDarwin [
+    "-I${lib.getDev libcxx}/include/c++/v1"
+  ];
 
   nativeBuildInputs = [
     setuptools-scm