Bundler.reset! nukes the global Definition.
CLI::auto_install performs the installation.

If requested by the environment variable and necessary, #auto_install will
automatically install any missing gems.
Autoinstall during the "show" command.
Autoinstall during the "binstubs" command.
Autoinstall during the "outdated" command.
Autoinstall for "bundle exec".
Autoinstall for "bundle open".
Autoinstall for "bundle console".
Autoinstall for "bundle license".
Autoinstall during "bundle clean".
Reset and restore BUNDLE_INSTALL in the specs.

Specs are still green with BUNDLE_INSTALL set to a nonempty value.