ChatOps responses broken with GitLab Runner 13.1

Summary

With GitLab Runner 13.1, ChatOps jobs no longer post the job output to Mattermost.

What is the current bug behavior?

In chat: /<trigger> run <job>

→ ephemeral message "Job has been started"

Job finishes

→ no reply in chat

What is the expected correct behavior?

In chat: /<trigger> run <job>

→ ephemeral message "Job has been started"

Job finishes

→ in channel message with job output

Possible fixes

The output for ChatOps jobs is by default the content of the build_script job stage (see lib/gitlab/chat/output.rb).

With GitLab Runner 13.1 (likely related to gitlab-runner#6409 (closed)) this seems to have been changed to step_script.