Skip ActionCable for Dummy Apps

We don't really use ActionCable for anything as far as I know. Having it
makes installing Turbo/Stimulus in extensions harder.

The installer for Turbo will look for a Gemfile in `Rails.root`, which is not
there in case of the dummy app, and fail. If, however, `config/cable.yml` can't be
found, the Turbo installer succeeds with a message that Turbo is
installed, just without the broadcasting feature (which is alright for
most apps I believe).

core/lib/generators/spree/dummy/dummy_generator.rb

@@ -44,6 +44,7 @@ def generate_test_dummy
       opts[:skip_yarn] = true
       opts[:skip_bootsnap] = true
       opts[:skip_javascript] = true
+      opts[:skip_action_cable] = true
 
       puts "Generating dummy Rails application..."
       invoke Rails::Generators::AppGenerator,