setup-ruby-flash 2.0: Optional main-bundle installation

🏷️ release setup-ruby-flash 🏷️ 2.0

Version 2.0 adds main-bundle-install so appraisal workflows can omit installation of the root Gemfile. It also corrects bundle-path export and retry handling for configured sources.

Read the upstream release notes for the complete changes and see the project repository for usage and installation.