head CI tests fail on needing tsort, re Ruby 3.6.0

[olleolleolle]

In the irb test, there was warning output, tripping up the test:

diff:
+ /home/runner/.rubies/ruby-head/lib/ruby/gems/3.5.0+2/gems/rbs-3.9.4/lib/rbs.rb:1
+ 1: warning: tsort was loaded from the standard library, but will no longer be pa
+ rt of the default gems starting from Ruby 3.6.0
+ You can add tsort to your Gemfile or gemspec to silence this warning.
  irb(main):001> :`
  => :`
  irb(main):002>

Perhaps it's in the rbs gem. EDIT: yes, tsort dependency was added to rbs in ruby/rbs@14fd824 - which is not yet released as a RubyGem.