gitflow: embed path to coreutils into the wrapper

gitflow is shell script and does not work without common tools (e.g readlink)
in PATH or with busybox instead of coreutils.

Busybox version of readlink does not support "-e" option. Probably it can be
replaced with "-f", but I didn't check.

pkgs/applications/version-management/git-and-tools/gitflow/default.nix

@@ -22,7 +22,8 @@ stdenv.mkDerivation rec {
   postInstall = ''
     wrapProgram $out/bin/git-flow \
       --set FLAGS_GETOPT_CMD ${pkgs.getopt}/bin/getopt \
-      --suffix PATH : ${pkgs.git}/bin
+      --suffix PATH : ${pkgs.git}/bin \
+      --prefix PATH : ${pkgs.coreutils}/bin
   '';
 
   meta = with lib; {