Support running bundle install with the --full-index option

[Skenvy]

I have circumvented an issue trying to install a recent gem that gives me an error

Downloading <gem-xyz> revealed dependencies not in the API or the lockfile

Where running the install with --full-index fixes this. I can't see a way to pass this option directly to the part of this action that runs the bundle install step, and so trying to use the action with the bundler-cache: true fails with the same error ~ revealed dependencies not in the API