Harden output filepaths for msys2 use

[hasufell]

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.

[hasufell]

@jasagredo @andreabedini @angerman