fix streaming track data_export.py bug

[KhylonWong]

Issue: the variable 'results' is being overwritten at the end of the loop, after it is potentially assigned values multiple times in the loop. Therefore, only the final result from the last runbook_path in streaming track will be considered for appending to dfs.

[maumueller]

Thanks @KhylonWong! I introduced that problem in 9b3a10a. I refactored your code so that the load/compute/prune chain is executed only once, I hope this improves readability of the code.