Better BINCI_TMP resolution on Windows?

```
PS C:\github\test-windows\node-sso-formatter> binci
√ Starting services vault, db
√ Running Task
‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧
sh: can't open '/tmp/binci.sh': No such file or directory
‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧‧
× Command failed
PS C:\github\test-windows\node-sso-formatter>
```

Not quite what I'd expect. What other information can I give to help debug this? cc @svetkomir

Reproduce the Windows PowerShell command shown with `binci` and inspect how `BINCI_TMP` is resolved before `/tmp/binci.sh` is opened. Done means the temporary script uses an available Windows-compatible location and the task runs without the shown missing-file error; additional environment details from the reporter may be needed to confirm the behavior.

Written by the indexing model from the issue text.