release-small: use unar instead of unrar

It's a free (SW) alternative. See #16868.

pkgs/top-level/release-small.nix

···
   time = linux;
   tinycc = linux;
   udev = linux;
-  unrar = linux;
+  unar = linux;
   unzip = all;
   upstart = linux;
   usbutils = linux;