This is mentioned in the developer notes, but isn't present in
`TSAN_OPTIONS`, resulting in:
```bash
WARNING: ThreadSanitizer: lock-order-inversion (potential deadlock) (pid=60508)
Cycle in lock order graph: M0 (0xffff98e02208) => M1 (0xffff98e0cbe8) => M2 (0xffff98e0cd98) => M0
<snip>
Hint: use TSAN_OPTIONS=second_deadlock_stack=1 to get more informative warning message
```
Add it, for (potentially) more informative output, when failures
occur. Checked that adding does output more information.