In Ruby 2.7, irb saves history to ~/.irb_history by default:
https://github.com/ruby/ruby/commit/078e50c5e9c53d935bdea6ca3ae156c0edcea467

This doesn't work well for Omnibus (and maybe other deployments), where
that won't be writable. So we disable irb history by default, but still
allow you to override it with your own ~/.irbrc.