Include the current scene name in a hidden setting of the Simple Blender
Render, and pass that as CLI argument to Blender.
This ensures that the correct scene is rendered when working directly on
shared storage (as that does not have a copy of the blend file per job).
The job setting `"scene"` is still optional. If it's missing or empty,
the `--scene <scene name>` CLI arg will simply not be passed to Blender.
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