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fb690300 - benchmarks/result_analyzer: set default output format to csv (#6033)

benchmarks/result_analyzer: set default output format to csv (#6033) To match our documentation. Moreover, note that using JSON output can lead to confusion: we read in all jsonl files in the output directory, and therefore after running the script twice, we try to read (and fail) the previously-generated jsonl file. With CSV this is not a problem.