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.
1 job for mnowak/add-step_script-stage in 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #971413
docker amd64
shellcheck

00:00:06