chore(nix): support build for experimental feature

[maiste]

Following the merge of #10724, I have written this PR so we are able to build the executable using Nix Flake. I'm far from being a Nix expert, so if someone with a better expertise is able to reduce the amount of code, it would be happy to change the code 😄

EDIT: Replace my solution with @gridbugs' one, as it is simpler and more elegant one. It consists in adding a function to the package default that could take some flags.

[gridbugs]

Heads up that I pushed a commit that fixes the failing nix test.

[gridbugs]

I pushed another change to remove the duplication of the experimental flags.

[emillon]

I think it's simpler and more idiomatic to use overrideAttrs to update the configureFlags of an existing derivation, I'll try something

[emillon]

how does it look @maiste ? I did a quick test and it seems to work but I'm not super familiar with the experimental flags so let me know if the resulting binaries work as you expect

[maiste]

On @gridbugs and my computer, the produced binary works as expected:

• No need to add some flags to get the compiler installed (no error about Ocaml when doing the dune pkg lock command)
• It displays the packages installed nicely when running dune build

EDIT: I didn't realize I didn't update the branch before trying. I retried with the musl version, and it worked! 👌

[gridbugs]

Nice work @emillon, I didn't know about overrideAttrs. I gave this a test on both macos-aarch64 and linux-x86_64 and both binaries have the features I expected.

[maiste]

I have rebased and cleaned the history. If the tests pass, I think it's ok to merge it 👍

[emillon]

Nice work @emillon, I didn't know about overrideAttrs

The part that made it click for me is this comment - derivations are mkDerivation calls and overrideAttrs changes the input of this function.

{ stdenv, bar, baz }: # this part gets overriden by `override`
stdenv.mkDerivation { # This part gets overriden by overrideAttrs
  pname = "test";
  version = "0.0.1";
  buildInputs = [bar baz];
  phases = ["installPhase"];
  installPhase = "touch $out";
}

[maiste]

@gridbugs @emillon, I don't have the merge right: can I gently ask one of you to do the merge? :)