treewide: move NIX_CFLAGS_COMPILE to the env attrset

with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper

this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure

pkgs/development/libraries/cpp-hocon/default.nix

@@ -15,7 +15,7 @@ stdenv.mkDerivation rec {
     sed -i -e '/add_subdirectory(tests)/d' lib/CMakeLists.txt
   '';
 
-  NIX_CFLAGS_COMPILE = "-Wno-error";
+  env.NIX_CFLAGS_COMPILE = "-Wno-error";
 
   nativeBuildInputs = [ cmake ];