fix(Summary workflows): don't overwrite previous results on new run

Currently the results for a participant are overwritten when the summary workflow is run again for the same participant, even if for a different challenge. We thought we had fixed this behavior previously, but that is only true if multiple challenges are specified in the *same* run.
I'm suspecting the overwrite is caused by our nextflow wiring, that somehow doesn't write new results into the existing folder for a participant, but instead creates the whole participant directory anew.