indexer.cli: Fix missing retries_left parameter

According to the task schema, that parameter should not be null and defaults to 0. But current scheduler backend implementation expects a value nonetheless and if it's null, that breaks the call.

Not necessarily for review, just to ensure it's fine jenkins wise.

This should fix [1]

• [1] https://jenkins.softwareheritage.org/job/DENV/job/tests/lastFailedBuild/artifact/swh-indexer.log

Test Plan

tox

---

Migrated from D4383 (view on Phabricator)