Add step_script stage
GitLab Runner 13.1.0 seems to add a step_script stage. Without this
stage recognized libvirt jobs fail with:
Executing "step_script" stage of the job script
Unsupported run stage 'step_script'The step_script stage is not documented upstream.
This was reported upstream as https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26418.