nixguix: Fix missing max_content_size constructor parameter

(Not really for review, just making sure my local unrelated failures are indeed local [2])

Fix [1]

• [1] https://sentry.softwareheritage.org/share/issue/4a01e6c4dfa04300b71ee117c1da3621/

• [2] chardet temporary conflict error once in a while... swh/meta$954

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Migrated from D5123 (view on Phabricator)