This is a 🐛 bug fix.

• ✅  I've added tests 🎉
• ✅  The test suite passes locally 🎉

Summary

Before telling bundler to load plugins in the :jekyll_plugins group, plugin_manager.rb checks for the existence of a Gemfile in the current working directory.
In some setups, the Gemfile may live elsewhere and be referenced using the BUNDLE_GEMFILE environment variable. In this case, the Gemfile check will fail and the :jekyll_plugins group will not be loaded.
Bundler is aware of the file (since the gems still resolve), so all we need to do is expand the check for a Gemfile to include the BUNDLE_GEMFILE environment variable.

Reproduction

For a given Jekyll site with plugins referenced in the Gemfile and not in the _config.yml, run the following in the parent directory:

export BUNDLE_GEMFILE=src/Gemfile 
bundle install
bundle exec jekyll build --source src

The plugins in the Gemfile plugins group will not be loaded.

Context

I haven't found an existing issue or PR referencing this.