stress: enable on darwin

pkgs/tools/system/stress/default.nix

···
   meta = with stdenv.lib; {
     description = "Simple workload generator for POSIX systems. It imposes a configurable amount of CPU, memory, I/O, and disk stress on the system";
     license = licenses.gpl2;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }