oscar: add Darwin support

[thefloweringash]

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.

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[Aleksanaa]

Also link executable to $out/bin and test if it works. (otherwise mainProgram becomes invalid)

[thefloweringash]

From a macOS perspective this seems like an unusual thing to do, and it would make more sense to me to remove the mainProgram attribute on Darwin. I haven't really been keeping up with nixpkgs lately, so if that's the convention then I don't mind doing so. The one thing I found when searching (mainProgram for Mac OS Apps) made it seem like macOS wasn't really considered when implementing the mainProgram feature, and also didn't arrive at a conclusion for what to do in this case.

[Aleksanaa]

I'm not very sure about that, but I got a request at #360354 (comment). As it doesn't hurt anything and it ease test when reviewing new versions, I'd suggest to add it as well.