opam: use aspcud on darwin

[winniequinn]

Motivation for this change

aspcud is the recommended solver for OPAM and the aspcud package is now building properly on Darwin. As such, we can remove the special case for Darwin that required OPAM to fall back to the built-in solver.

Note: The failure related to the csv package mentioned in #29336 has been fixed and nox-review now passes.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

[mention-bot]

@winniequinn, thanks for your PR! By analyzing the history of the files in this pull request, we identified @aske, @hakuch and @henrytill to be potential reviewers.