message: add cmake_minimum_required to vcpkg_tags
  this allows people to use, for example, IN_LIST
diff: @@ -75,9 +75,11 @@ namespace vcpkg::CMakeVars
  const std::map<Triplet, int>& emitted_triplets)
  {
  const auto& fs = paths.get_filesystem();
  - std::string extraction_file("macro(vcpkg_triplet_file VCPKG_TRIPLET_ID)\n");
  + std::string extraction_file;
  Strings::append(extraction_file,
  + "cmake_minimum_required(VERSION 3.5)\n"
  + "macro(vcpkg_triplet_file VCPKG_TRIPLET_ID)\n",
  "set(_vcpkg_triplet_file_BACKUP_CURRENT_LIST_FILE \"${CMAKE_CURRENT_LIST_FILE}\")\n");
  for (auto&& p : emitted_triplets)
  