Set base path to hub url for canonical datasets
This should allow canonical datasets to use relative paths to download data files from the Hub

cc @polinaeterna this will be useful if we have audio datasets that are canonical and for which you'd like to host data files
If we agree to have data files in a dedicated directory "data/" then we should be fine. You're right we should not try to edit a dataset script from the repository directly, but from github, in order to avoid conflicts