Start task fails for preempted nodes because unzip prompts for overwrite

Some clusters with preempted low pri nodes are having their start task fail when the node comes back:

```
replace aztk/client.py? [y]es, [n]o, [A]ll, [N]one, [r]ename:  NULL
(EOF or read error, treating as "[N]one" ...)
```

We need to pass the `-o` overwrite flag to all unzip commands in the start task.