Fix; enqueuing a job that has already been enqueued

[Jamesbarford]

Fixes the error message below which happens when trying to enqueue a job that has already been enqueued. We only use the return value from this query in tests.

[2025-09-24T08:51:57Z ERROR site::job_queue] Cron job failed to execute: failed to insert benchmark_job

    Caused by:
        query returned an unexpected number of rows

[Kobzol]

Right, because we can enqueue parent jobs where the jobs were already computed.