Hi Alex,
That’s an interesting one - since chruby is now enabled in the image, it is detecting the .ruby-version in your checkout and trying to select it. If you change the .ruby-version in your repo to 2.6 you should be OK. We have 2.6.3 installed in the image, so you could change to that either.
The Xcode 11.1 GM image has Ruby 2.6.4: Xcode 11.1 Image released
Marc