casperjs: do not move the current working directory

If /tmp and /nix are in different filesystems, this causes the `find`s
in the fixup phase to fail because of a stale file handle:

    find: cannot get current directory: No such file or directory

pkgs/development/tools/casperjs/default.nix

@@ -28,9 +28,8 @@ in stdenv.mkDerivation rec {
     make test
   '';
 
-
   installPhase = ''
-    mv $PWD $out
+    cp -r . $out
   '';
 
   meta = {