cpeditor: fixup build

cpeditor require c++14, the setting of CMAKE_CXX_STANDARD does not seem to affect submodules

pkgs/applications/editors/cpeditor/default.nix

···
     substituteInPlace src/Core/Runner.cpp --replace "/bin/bash" "${runtimeShell}"
   '';
 
+  env.NIX_CFLAGS_COMPILE = "-std=c++14";
+
   meta = with lib; {
     description = "An IDE specially designed for competitive programming";
     homepage = "https://cpeditor.org";