save_result/export_workspace: correctly handle derived-from links

[jdries]

"Derived from" data is not available in collection.json with mutiple save_results
The issue here is specific for multiple save_results, leading to the "Derived from" data not being available in collection.json.
It seems the collection.json is overwritten with the last save_result node, showing only info on GAMMA0-NB-OBS in this case.
Example: j-2502131919384d77ab37566a57065693