scenario: Fix action variable name

This was always meant to be `on-message=eos` (like in fast_forward.scenario)

Fixes #58

Part-of: <gstreamer/gst-devtools!254>
35 jobs for !254 with backport-mr-240-into-1.18-action-message in 23 minutes and 56 seconds (queued for 15 seconds)
merge request