Harden output filepaths for msys2 use

`cabal list-bin` on msys2 may sometimes produce paths like:

```
C:/Users/hasuf/git/stable-haskell/ghc/_build/stage0\build\x86_64-windows\ghc-9.8.4\cabal-install-3.17.0.0\x\cabal\build\cabal\cabal.exe
```

These can't be intepreted in an msys2 shell. But these two variants work:

1. `C:\\foo\\bar`
2. `C:/foo/bar`

Since forward slash is supported everywhere in windows, my suggestion would be to default to forward slash for output filepaths.

Start at the `cabal list-bin` command and trace how output paths are formatted on MSYS2. Compare its mixed-separator output with the two working Windows forms in the issue; done means the command emits a path usable from an MSYS2 shell without breaking native Windows use.

Written by the indexing model from the issue text.