Use different nixpkgs for rust and ocaml toolchains

flake.nix

@@ -13,7 +13,8 @@
     };
     charon = {
       url = "github:aeneasverif/charon";
-      inputs.nixpkgs.follows = "eurydice/nixpkgs";
+      inputs.nixpkgs.follows = "nixpkgs"; # Recent nixpkgs for rust-overlay
+      inputs.nixpkgs-ocaml.follows = "eurydice/nixpkgs"; # eurydice nixpkgs for ocaml compat
       inputs.flake-utils.follows = "flake-utils";
       inputs.rust-overlay.follows = "rust-overlay";
       inputs.crane.follows = "crane";
@@ -26,11 +27,6 @@
     };
     eurydice = {
       url = "github:aeneasverif/eurydice";
-      # If we override this, we would need to override karamel's nixpkgs too to
-      # get compatible ocaml versions, but flakes don't support nested
-      # overrides. We also can't use eurydice's nixpkgs everywhere because it
-      # does not contain `mold-wrapped` which is required by libcrux.
-      # inputs.nixpkgs.follows = "nixpkgs";
       inputs.flake-utils.follows = "flake-utils";
       inputs.charon.follows = "charon";
     };