Dune 3 cannot delete files named `NUL` on Windows

When there's a cycle in dependencies (I guess) or for some other reason a NUL file can be produced in the _build directory. Dune fails to remove it. It can be tricky to remove them: https://stackoverflow.com/questions/17883481/delete-a-file-named-nul-on-windows.

Error: Conflict between the following libraries:
- "lwt.unix" in _build/default/src/unix
- "lwt.unix" in
  "C:\\Users\\antonin\\.opam\\ocaml-variants.4.13.1+mingw64c\\lib\\lwt\\unix"
  -> required by library "lwttester" in _build/default/test
-> required by executable main in test/ppx/dune:2
-> required by _build/default/test/ppx/main.exe
-> required by %{exe:main.exe} at test/ppx/dune:10
-> required by alias test/ppx/runtest in test/ppx/dune:7
$ dune clean
Error: rmdir(_build\default\src\unix): Directory not empty
$ ls _build/default/src/unix
NUL

Start by reproducing the issue on Windows with the dependency-cycle scenario and dune clean, focusing on _build/default/src/unix/NUL. The issue names no source file or test; done means cleanup removes the NUL entry and completes without the directory-not-empty error.