Fix an error path in job queue worker's RunTask

In case the job fails, we try to set the job's run_op_idx to -1.
However, this is a wrong variable, which wasn't detected until the
__slots__ addition. The correct variable is run_op_index.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>