Consider portable `System.FilePath` from `filepath-1.4.100.4`+

[sorki]

Sometimes we concat paths, or we need a normalized StoreDir, as currently it is /nix/store and it's easy to forget to add <> "/" <>. Also too POSIXy.

• https://hasufell.github.io/posts/2022-06-29-fixing-haskell-filepaths.html
• https://hackage.haskell.org/package/filepath-1.4.100.4

[Ericson2314]

See also NixOS/nix#9205 upstream