cvs.tasks: Fix type

This fixes build [1]

Note that this function should definitely be moved in swh.core because it's redundant with other definitions in other loaders.

Let's unstuck the build first.

• [1] https://jenkins.softwareheritage.org/view/swh-draft/job/DLDSVN/job/tests/1304/console

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Migrated from D6670 (view on Phabricator)