Windows: Error: CreateProcess(): No such file or directory when PATH contains unicode characters

This is all explained in https://github.com/janestreet/spawn/issues/57

## Specifications

- Version of `dune` (output of `dune --version`): 3.5.0 (but the same code is used in the latest version)
- Operating system (distribution and version): Windows (using Cygwin Terminal but it shouldn't matter)