nix: cross compile dune to windows

[Alizter]

This derivation allows us to cross-compile Dune to windows using mingw. This can be used to distribute Windows binaries for https://github.com/ocaml-dune/binary-distribution/

[rgrinberg]

Nice. Shall we add this to CI as well? Seems like this should be a lot faster.

Is it all possible to run our test suite with wine?

[Alizter]

Nice. Shall we add this to CI as well? Seems like this should be a lot faster.

Is it all possible to run our test suite with wine?

I can add some build jobs to CI so we can test it actually continues to work going forward however it will take more work to do something useful with the binary.

For cram tests there are different levels of integration that are possible. We could wrap each cross-windows binary with wine and run it in regular bash or run windows bash in wine together with all the windows executables that bash will exec.

The second option seems better since it will more closely match the behaviour of running tests on windows.