darwin: Disable some more packages that have never built

streamripper: https://hydra.nixos.org/build/61726659
racket: https://hydra.nixos.org/build/61750850
radare: https://hydra.nixos.org/build/61756146
stockfish: https://hydra.nixos.org/build/61726172

pkgs/applications/audio/streamripper/default.nix

···
     homepage = http://streamripper.sourceforge.net/;
     description = "Application that lets you record streaming mp3 to your hard drive";
     license = licenses.gpl2;
-    platforms = platforms.unix;
+    platforms = platforms.linux;
     maintainers = with maintainers; [ the-kenny ];
   };
 }

pkgs/development/interpreters/racket/default.nix

···
     homepage = http://racket-lang.org/;
     license = licenses.lgpl3;
     maintainers = with maintainers; [ kkallio henrytill vrthra ];
-    platforms = platforms.x86_64;
+    platforms = [ "x86_64-linux" ];
   };
 }

pkgs/development/tools/analysis/radare/default.nix

···
     homepage = http://radare.org/;
     license = stdenv.lib.licenses.gpl2Plus;
     maintainers = with stdenv.lib.maintainers; [viric];
-    platforms = with stdenv.lib.platforms; all;
+    platforms = with stdenv.lib.platforms; linux;
   };
 }

pkgs/games/stockfish/default.nix

···
       much stronger than the best human chess grandmasters.
       '';
     maintainers = with maintainers; [ luispedro peti ];
-    platforms = with platforms; i686 ++ x86_64;
+    platforms = ["x86_64-linux" "i686-linux"];
     license = licenses.gpl2;
   };