TIL: Bundler's job parallelization uses threads, not processes

https://github.com/rubygems/rubygems/blob/d57d302cbb265f5164b0bc448191e9beec257c43/bundler/lib/bundler/worker.rb#L90

The default is "the number of available processors", but the number of processors has nothing to do with the optimal number here, only 1 processor will be used.