I've only used circleci and github actions for this, but in both cases precompiled rubies are available. On circleci you run your tests on a ruby docker image with the right version of ruby installed. On Github Actions I use https://github.com/ruby/setup-ruby, which installs an compiled ruby in a second or two, and also properly caches your gems. I think if someone's CI environment is building ruby from source they are doing it wrong.