yarn called in swh-search setup.py but not present in developer setup docs

When following the instructions here: https://docs.softwareheritage.org/devel/developer-setup.html

The pip install step fails when it attempts to install swh-search, because it calls yarn which isn't installed on the machine. Either this should be made optional, or the yarn dependency should be added to the base developer setup.

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Migrated from T3742 (view on Phabricator)