Build the binaries for windows

[gridbugs]

We don't need to wait for dune package management to work on windows to start publishing dev preview builds of it.

[shonfeder]

@Alizter reported today that he has started substantial work on this, so this is now in progress and we should ensure it gets completed smoothly! Note that completing this doesn't require #140 .

To complete this issue, the work started on building the binaries for windows should be landed on this repo and the binaries made available via get.dune.build.

[Alizter]

To add some more information on what I have so far:

• I have added cross-mingw support to https://github.com/nix-ocaml/nix-overlays/ building on earlier attempts. This allows us to cross compile from a linux host to a windows target. We can use my fork for the time being until we upstream the changes pending further review.
• There are some tweaks I need to make to dune's bootstrapping process, namely allowing us to override the ocamlc -config values that it detects in order to substitute target system values.

Here is what we need to do:

• We need to set up a linux CI runner that targets Windows and inspects the binary header as a sanity check. This differs from the existing runenrs where the host and target are the same system. This may also allow for future improvement of the existing runners if the cross support can do linux x86 -> macos arm etc. Allowing for a faster deploy.
• The Windows binaries may need some additional meta-data to work well in the Windows eco-system. I am not familiar with what these may be but it would need to be researched and considered.
• A future improvement would be for a Windows installer script to be shipped instead which would download and install the binary instead.

[shonfeder]

Ali reports having a patch set ready for a PR into the binary-distribution.

[Alizter]

Correction: A patch set ready for nix-overlays which needs

1. to be referenced from dune's flake
2. then used in binary-distribution

[Alizter]

Here is the PR for nix-overlays:

• add mingw support nix-ocaml/nix-overlays#2395

Afterwards we need a PR to update the dune flake and provide cross compilation outputs:

• nix: cross compile dune to windows ocaml/dune#15285

Afterwards we can implement the distribution here.