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

[ninsch3000]

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.

[ninsch3000]

@dominikburri , if you happen to figure it out, great, if not, not the end of the world, we'll then just add the challenge name to the participant root directory and work with that directory structure.

[ninsch3000]

I don't know whether the described behavior still happens with the current version of the SWFs. Please check and either close this issue or comment.