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

layus/nix,NixOS/nix,ehmry/nix,peti/nix,vcunat/nix,ehmry/nix,dtzWill/nix,zimbatm/nix,shlevy/nix,ehmry/nix,dezgeg/nix,layus/nix,dezgeg/nix,vcunat/nix,ehmry/nix,peti/nix,dezgeg/nix,layus/nix,pikajude/nix,vcunat/nix,rycee/nix,NixOS/nix,layus/nix,dtzWill/nix,NixOS/nix,shlevy/nix,dezgeg/nix,peti/nix,NixOS/nix,zimbatm/nix,zimbatm/nix,bennofs/nix,bennofs/nix,vcunat/nix,dtzWill/nix,bennofs/nix,rycee/nix,zimbatm/nix,peti/nix,peti/nix,pikajude/nix,rycee/nix,NixOS/nix,shlevy/nix,dezgeg/nix,layus/nix,shlevy/nix,rycee/nix,dtzWill/nix,rycee/nix,shlevy/nix,ehmry/nix,bennofs/nix,pikajude/nix,dtzWill/nix,ehmry/nix