From #12946 (comment)

I think we need to make it obvious to others that may not be experienced with bash what is going on here. As GitHub had their special set-output command that we replaced, we should create a script to use for appending multiple lines to workflow output. Please create an issue on this, I propose we do something like ./append-to-github-output.sh message.

This will also help in future maintainence/debugging efforts, since we can check for existence of message, etc.