nixguix: Add support for downloads over FTP
python-requests does not support download via FTP by default, so this causes errors like this one: https://sentry.softwareheritage.org/share/issue/96e6c7c0442e4343919d510c9a0fc977/
This could possibly be done using https://pypi.org/project/requests-ftp/
Migrated from T2687 (view on Phabricator)