staging: Running nixguix on guix sources

Scheduling command:

swhscheduler@scheduler0:~$ swh scheduler --config-file /etc/softwareheritage/scheduler.yml task add load-nixguix url=http://guix.gnu.org/sources.json
WARNING:swh.core.cli:Could not load subcommand storage: No module named 'swh.journal'
INFO:swh.core.config:Loading config file /etc/softwareheritage/scheduler.yml
Created 1 tasks

Task 1224855
  Next run: just now (2020-07-06 14:03:59+00:00)
  Interval: 1 day, 0:00:00
  Type: load-nixguix
  Policy: recurring
  Args:
  Keyword args:
    url: 'http://guix.gnu.org/sources.json'

From irc ping:

15:39 <zimoun> lewo: Hi, the sources.json for Guix is served at http://guix.gnu.org/sources.json so could you add to your staging for testing?
15:40 <+olasd> nice!
15:40 <+olasd> ardumont: ^
15:41 <lewo> zimoun: yeah! Cool ;)
15:42 <+ardumont> ack on this, nice!
15:42 <+ardumont> lemme finish my stuff first ;)

---

Migrated from T2485 (view on Phabricator)