make sure doc and manpage outputs are symlinks

Part of NixOS/nixpkgs#392549
The doc and manpage fix already happend in 0ddfbc5

(cherry picked from commit 15dfeb9)

Author: Kirens

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 = {