racket: disable on darwin

[henrytill]

Motivation for this change

Darwin builds of Racket are segfaulting for unknown reasons:
https://hydra.nixos.org/build/49778947

I don't have the capacity to troubleshoot this issue ATM. Anyone with the capacity is more than welcome to try to fix the build, but until such a person steps forward, I am proposing that we simply disable builds on Darwin.

Aside from updating the meta.platforms attribute, I have kept other bits of Darwin-specific code intact so that they will not be lost to the sands of time.

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 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.

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[mention-bot]

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

[LnL7]

I personally prefer not to change the platforms for packages that should work.

[LnL7]

This should be fixed by #22571.

[henrytill]

This should be fixed by #22571.

A much better solution!