fix(coderd/database): prevent AcquireProvisionerJob from grabbing canceled jobs (#21852)

The AcquireProvisionerJob query only checked started_at IS NULL, allowing
it to acquire jobs that were canceled while pending (which have
completed_at set but started_at still NULL).

Added completed_at IS NULL check to the query to prevent this.

Also fixed JobCompleteBuilder.Do() in dbfake to set started_at when
completing jobs to match production behavior.

Fixes coder/internal#1323

coderd/database/queries.sql.go

@@ -10251,6 +10251,7 @@ WHERE
 			provisioner_jobs AS potential_job
 		WHERE
 			potential_job.started_at IS NULL
+			AND potential_job.completed_at IS NULL
 			AND potential_job.organization_id = $3
 			-- Ensure the caller has the correct provisioner.
 			AND potential_job.provisioner = ANY($4 :: provisioner_type [ ])