Motivation for this change

https://github.com/ocamllabs/dune-release/releases/tag/1.4.0

Things done

Required a bit of fiddling around to get the mdx tests to work, but seems good now. yes for some reason would complain about a broken pipe in the no_doc test which I couldn't reproduce outside of nix-build. Seems to be safe to just ignore by piping it to /dev/null which makes the output match up again. No idea what's causing this.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.