Customize the deleter to deallocate a 2D array through std::unique_ptr

Suppose I am parsing an environment variable list from a given map<string, string> to a 2D memory hold by unique_ptr<char*[]>. However, I am not sure how I can customize the deleter for this 2D memory case.

// Given: env (type of map<string, string>)
// Return: unique_ptr<char*[]> (with customized deleter)

// Prepare for parsing the environment to c-style strings
auto idx = size_t{0};

// What should I fill for `ret` a proper deleter that won't give memory leak?
auto ret = std::make_unique<char*[]>(env.size() + 1, ???);   
for(const auto& kvp : env) {
  auto entry = kvp.first + "=" + kvp.second;
  ret[idx] = new char[entry.size() + 1]; 
  strncpy(ret[idx], entry.c_str(), entry.size() + 1); 
  ++idx;
}
ret[idx] = nullptr;  // For the later use of exec call

return ret;

Apparently, the above code leaks, because of the new operator in the inner for loop.