snapshot/metadata inversion in origin_visit_status_get_random

Discovered during the tests around swh/infra/sysadm-environment#3517 (closed)

Relative sentry issue: https://sentry.softwareheritage.org/share/issue/47852ebc29d74a03beba720479d48ed3/

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Migrated from T3539 (view on Phabricator)