Wrong release for DNS Challenge 2020

## 🐛 Bug
Regard using the DNS-Challenge baseline, there are issues when using the newest version of the DNS Challenge repo, where the main problems occur when evaluating models, as the testdata has been removed from the repo newest release of the DNS Challenge repo.

I suggest that in line 9 in `asteroid/blob/master/egs/dns_challenge/baseline/local/download_data.sh` is changed from `git clone https://github.com/microsoft/DNS-Challenge` to `git clone -b interspeech2020/master https://github.com/microsoft/DNS-Challenge` so it clones the last stable release of the DNS Challenge 2020 repo.

Open egs/dns_challenge/baseline/local/download_data.sh and inspect line 9, then run the download step to reproduce which DNS-Challenge revision is fetched. Done means the script clones the interspeech2020/master branch and the DNS Challenge testdata is available for model evaluation.

Written by the indexing model from the issue text.