libunistring: use gnu libiconv on darwin

[paparodeo]

libunistring doesn't appear to be used in the stdenv build and thus if it is modified will result in 59k rebuilds. Removing from stdenv reduces rebuilds if libunistring is modified to 37k (tho this PR will still incur 58k rebuilds). of the 37k rebuilds most are due to libpsl / libidn2 and to a lesser amount gnutls and guile.

https://lists.gnu.org/archive/html/bug-gnulib/2024-05/msg00375.html macoOS libiconv 14 & 15 do not work with libunistring and a configure test added in 1.3 rejects even building with it so use gnu libiconv on darwin

reverts 76cca7e as it is now unnecessary

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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.

---

Add a 👍 reaction to pull requests you find important.