agedu: expand platforms to unix

pkgs/tools/misc/agedu/default.nix

···
     homepage = "https://www.chiark.greenend.org.uk/~sgtatham/agedu/";
     license = licenses.mit;
     maintainers = with maintainers; [ symphorien ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }