haskellPackages.with-utf8: fix build on x86 darwin.

[d-xo]

Description of changes

Messed up my rebase onto haskell-updates in #327204 and triggered a mass notification 😳. Resubmitting here with the correct diff.

This should fix a build error (missing libchatset.h) for with-utf8 that manifests on x86 darwin only (hydra. The first build that started failing here was #259447667, which contains some changes to the libiconv package (#301354) that may be the root cause of the regression.

The pr here simply conditionaly adds libiconv to extraLibraries for with-utf8 on x86 darwin only, and doesn't dig deeper into the stdenv changes (which are unfortunately a little out of my league).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[maralorn]

Thx!