Fix bundler isolation for generated example apps

Use BUNDLE_GEMFILE to explicitly point to the generated app's Gemfile
when running rake tasks. This prevents bundler from using gems from a
parent workspace's vendor/bundle, which could have different versions.

This fixes the Shakapacker version mismatch error in CI where the
generated app had shakapacker 8.2.0 installed but Rails was loading
shakapacker 9.4.0 from the parent react_on_rails vendor/bundle.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

react_on_rails/rakelib/shakapacker_examples.rake

@@ -113,8 +113,11 @@ namespace :shakapacker_examples do # rubocop:disable Metrics/BlockLength
 
       sh_in_dir(example_type.dir, "npm install")
       # Generate the component packs after running the generator to ensure all
-      # auto-bundled components have corresponding pack files created
-      sh_in_dir(example_type.dir, "bundle exec rake react_on_rails:generate_packs")
+      # auto-bundled components have corresponding pack files created.
+      # Use BUNDLE_GEMFILE to ensure bundler uses the generated app's Gemfile
+      # and not any parent workspace's bundle, which could have different gem versions.
+      gemfile_path = File.join(example_type.dir, "Gemfile")
+      sh_in_dir(example_type.dir, "BUNDLE_GEMFILE=#{gemfile_path} bundle exec rake react_on_rails:generate_packs")
     end
   end