Resolve an ambiguous call issue setting nlohmann::json value from vtkStdString

On the Intel oneAPI 2021 compilers, this call was leading to compilation
error due to an ambiguous call to __is_path_src:

bits/fs_path.h:119:16: error: call to '__is_path_src' is ambiguous

    : decltype(__is_path_src(std::declval<_Source>(), 0))

Resolved by statically casting the string to a std::string

IO/CellGrid/vtkCellGridWriter.cxx

@@ -201,7 +201,7 @@ nlohmann::json serializeArrayValues(vtkAbstractArray* arr)
   {
     for (vtkIdType ii = 0; ii < arr->GetNumberOfValues(); ++ii)
     {
-      result.push_back(arr->GetVariantValue(ii).ToString());
+      result.push_back(static_cast<std::string>(arr->GetVariantValue(ii).ToString()));
     }
   }
   return result;