ddnet-server: fix Darwin build

The server variant of the ddnet package does not ship the client, so we
don't need to patch in libsteam_api.dylib

Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>

pkgs/by-name/dd/ddnet/package.nix

@@ -111,7 +111,7 @@ stdenv.mkDerivation rec {
     rm -rf $out/share/metainfo
   '';
 
-  preFixup = lib.optionalString stdenv.hostPlatform.isDarwin ''
+  preFixup = lib.optionalString (stdenv.hostPlatform.isDarwin && buildClient) ''
     # Upstream links against <prefix>/lib while it installs this library in <prefix>/lib/ddnet
     install_name_tool -change "$out/lib/libsteam_api.dylib" "$out/lib/ddnet/libsteam_api.dylib" "$out/bin/DDNet"
   '';