`bundler update --bundler` is hardcoded to use rubygems.org as source

[shanecav84]

When running bundle install, if the bundler version does not match the one in the Gemfile, bundler will install a version of itself to match the Gemfile version. I would expect this self-update to use the Gemfile source to pull it's updated version, but it is hardcoded to rubygems.org

https://github.com/rubygems/rubygems/blob/ade324bdc8ea77b342f203cb7f3929a456d725ed/bundler/lib/bundler/self_manager.rb#L127

I'd like to be able to use https://gem.coop as a source without having to use bundle config set --global mirror.https://rubygems.org https://gem.coop. Can the source be determined from gem sources or the Gemfile source?