GNU lister/loader: no recent visits for origins that have definitely received updates

(tentatively putting this under the lister label)

Looks like the gnu lister is missing some updates. For instance, GDB (https://ftp.gnu.org/gnu/gdb/) has had a bunch of releases in 2020, but our last visit to it dates back to december 2019 (https://archive.softwareheritage.org/api/1/origin/https://ftp.gnu.org/gnu/gdb//visits/).

The upstream https://ftp.gnu.org/tree.json.gz shows the recent files pretty clearly.

According to the scheduler database the last run of the lister was on 2020-12-07, and was "eventful".

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Migrated from T2875 (view on Phabricator)