Rspec - How can I stub a constant that is defined in config/environment.rb?

I have defined a constant in config/environment.rb called LOCAL_SETTINGS that I use for configuration and use throughout my app. It's a YAML file stored in config/local_settings.yml and will sometimes contain sensitive data like API keys etc.

I'm currently trying to write a spec for a method that uses LOCAL_SETTINGS["slack"]["slack_token"].

The problem is that the constant is not being stubbed for my expectations. ie expect(subject.path).to eq(help_request) fails because it returns a path including the non-stubbed LOCAL_SETTINGS hash.

However if I put a debugger in beneath stub_const and then type LOCAL_SETTINGS, I can see that stub_const has worked.

My questions are:

1. Is there something I can do in Rspec to get stubs working for constants defined in config/environment.rb?
2. Should I simply define this constant elsewhere? If so, where? I will need access to it throughout my application in the app/ lib/ and spec/ folders.

My config/environment.rb file:

# Load the Rails application.
require_relative 'application'

LOCAL_SETTINGS = YAML.load_file("#{Rails.root}/config/local_settings.yml")

# Initialize the Rails application.
Rails.application.initialize!

My spec:

describe RequestBuilder, type: :model do
  let(:help_params) { {"user_name"=>"some_user_name", "text"=>"release-bot help"} }
  let(:help_builder) { RequestBuilder.new(help_params) }
  let(:help_request) { "/api/files.upload?file=lib%2Fresponses%2Fhelp&filetype=ruby&channels=stubbed_channel&token=stubbed_token" }
  let(:slack_settings) { {"slack"=>{"slack_token"=>"stubbed_token", "slack_channel"=>"stubbed_channel"}} }

  context 'Given an incoming request' do
    context 'With a correctly formatted request' do
      context 'And the "help" command' do

        subject { help_builder.build_request_hash }

        it 'builds a request containing the help data' do
          stub_const("LOCAL_SETTINGS", slack_settings)

          expect(subject).to have_key(:request)
          expect(subject[:request].path).to eq(help_request)
        end
      end
    end
  end
end