Fix fsaverage across datasets

[gemeinl]

you should not need this. You can call this function:

https://mne.tools/stable/generated/mne.datasets.fetch_fsaverage.html

to have fsaverage in the derivatives folder. It's much cleaner indeed to decouple datasets

_Originally posted by @agramfort in https://github.com/dengemann/meeg-brain-age-benchmark-paper/pull/32#discussion_r749685505_