make sure doc and manpage outputs are symlinks

Part of https://github.com/NixOS/nixpkgs/pull/392549
The doc and manpage fix already happend in 0ddfbc5939

(cherry picked from commit 15dfeb9182)

packaging/everything.nix

@@ -201,8 +201,8 @@ stdenv.mkDerivation (finalAttrs: {
       done
 
       # Forwarded outputs
-      ln -s ${nix-manual} $doc
-      ln -s ${nix-manual.man} $man
+      ln -sT ${nix-manual} $doc
+      ln -sT ${nix-manual.man} $man
     '';
 
   passthru = {