Potential multiprocessing issue on Windows with 64+ cores

Seeing the following exceptions on a Windows machine with 64 cores when running trailrunner via ufmt:

```
ValueError: need at most 63 handles, got a sequence of length 63
```

Setting `max_workers=X` where X < 60 seems to be a solution, but should verify that setting `max_workers=60` doesn't mean we spawn 60 child processes even if there are fewer jobs to complete. 