Had the same problem.
It’s indeed related to this https://github.com/Homebrew/brew/pull/3240/files sent above.
Just insert a - brew update on your dependencies: pre: and you will be fine.
It will, as informed in a comment in this PR, “unset HOMEBREW_RUBY_PATH to ensure that this doesn’t “stick” on a Ruby 2.0 version after a portable Ruby has been installed.”.
You can continue to use the 8.2 image, also there’s no need to change the Ruby version.
dependencies:
pre:
- brew update