unpack-channel.nix: fix 'mv' corner case

unpack-channel.nix fails if the tarball contains a directory named the
same as the channel:

  mv: cannot move 'nixpkgs' to a subdirectory of itself, '.../nixpkgs'

This commit fixes that by not moving the directory if it already has the
correct name.

corepkgs/unpack-channel.nix

@@ -15,7 +15,9 @@ let
       else
         ${bzip2} -d < $src | ${tar} xf - ${tarFlags}
       fi
+      if [ * != $channelName ]; then
         mv * $out/$channelName
+      fi
       if [ -n "$binaryCacheURL" ]; then
         mkdir $out/binary-caches
         echo -n "$binaryCacheURL" > $out/binary-caches/$channelName