don't have tar depend upon tar

svn path=/nixpkgs/trunk/; revision=1577

pkgs/stdenv/nix-linux-static/gnutar-static/builder.sh

@@ -1,5 +1,4 @@
 . $stdenv/setup
  
-tar zxvf $src
 mkdir $out
-cp -a tar-1.13.25/* $out
+cp -a $src $out

pkgs/stdenv/nix-linux-static/gnutar-static/default.nix

@@ -3,5 +3,5 @@
 stdenv.mkDerivation {
   name = "gnutar-static-1.13.25";
   builder = ./builder.sh;
-  src = ./tar-1.13.25-static.tar.gz;
+  src = ./tar;
 }