Investigate why GitHub fork detection did not bring a speed-up

GitHub fork detection started running on 2022-04-28 around 19:00 UTC, but there is no noticeable speedup around that time:

• https://grafana.softwareheritage.org/d/b_xh3f9ik/worker-task-processing?orgId=1&viewPanel=65&from=1650837600000&to=1651528799000
• https://grafana.softwareheritage.org/d/Cgi8dR8Wz/ingestion-status?orgId=1&from=1650837600000&to=1651528799000

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Migrated from T4219 (view on Phabricator)