build: set gem location as vendor/bundle

[iBotPeaches]

The gitignore file already has vendor/bundle ignored - a common location to install gems.

Without a sane default set - folks like me who are trying to learn all the pipelines cannot effectively run them. Take this example of running bundle exec fastlane update_docs on main fastlane repo.

Ultimately after fastlane clones a new docs repo it executes this.

Bundler.with_clean_env do
  sh("bundle update")
end

Which of course fails because the default gem location for this system at least is not in a user-land area - its system.

Errno::EACCES: Permission denied @ dir_s_mkdir - /var/lib/gems/3.3.0/cache

I'm under strong belief projects should declare where gems should be stored so alignment between local usage, CI usage and command usage can all depend on a specific setup. A simple bundle update hard-coded requires the end user to preconfigure environment variables or more - when the project should decide that.