Currently, `pkgs.ruby` is ruby 2.7.6, which is in 'security updates
only' mode and scheduled to go EOL April 2023. Updating `pkgs.ruby`
to be ruby 3.1 would be a rather large change, but using ruby 3 to
bootstrap building ruby itself would be a nice first step in that
direction.
Tested with asciidoctor that this indeed now allows building a ruby
application without getting ruby 2.7 in the build closure.
Also fixes#192729
c5e2f3c066