Let apt decide unpack order instead of sorting filenames

Now that the deb files can reside in different places sorting them leads
to subtil differences in the order and hence the created chroot. apts
unpack order on the other hand might not be a good order (but why would
one sorted from a to z be one?), but it is far more stable as it is
independent on the filenames.

mmdebstrap

@@ -2150,9 +2150,6 @@ sub run_download() {
         }
     }
 
-    # Unpack order matters for e.g. timestamps on directories
-    @essential_pkgs = sort @essential_pkgs;
-
     return (\@essential_pkgs, \@cached_debs);
 }