apt-offline: set platforms to linux

(Only tested on Linux; might work on other systems.)

pkgs/tools/misc/apt-offline/default.nix

···
     description = "offline APT package manager";
     license = licenses.gpl3;
     maintainers = [ maintainers.falsifian ];
-    platforms = platforms.all;
+    platforms = platforms.linux;
   };
 }