Fix rubocop binstub to avoid loading configs from parent directories

Based on rubygems/rubygems#6915

This occurs especially when generating a new rails app within a rails checkout:
https://github.com/rails/rails/actions/runs/7360317953/job/20036287021#step:4:171

railties/lib/rails/generators/rails/app/templates/rubocop.tt

@@ -1,27 +1,12 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
-#
-# This file was generated by Bundler.
-#
-# The application 'rubocop' is installed as part of a gem, and
-# this file is here to facilitate running it.
-#
-
 ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
 
-bundle_binstub = File.expand_path("bundle", __dir__)
-
-if File.file?(bundle_binstub)
-  if File.read(bundle_binstub, 300).include?("This file was generated by Bundler")
-    load(bundle_binstub)
-  else
-    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
-Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
-  end
-end
-
 require "rubygems"
 require "bundler/setup"
 
+# explicit rubocop config increases performance slightly while avoiding config confusion.
+ARGV.unshift("--config", File.expand_path("../.rubocop.yml", __dir__))
+
 load Gem.bin_path("rubocop", "rubocop")