Fix Linux build & some warnings (#6438)

* Fix linux deps debug build

* Use the same DL_CACHE for release build when building debug version of deps on Linux.
This prevents downloading the same source packages twice, and avoid downloading again after deleting the build dir.

* Fix debug build

* Fix warnings "loop variable creates a copy from type" and "loop variable binds to a temporary constructed from type"

src/libslic3r/Model.cpp

@@ -3538,7 +3538,7 @@ void check_model_ids_validity(const Model &model)
         for (const ModelInstance *model_instance : model_object->instances)
             check(model_instance->id());
     }
-    for (const auto mm : model.materials) {
+    for (const auto& mm : model.materials) {
         check(mm.second->id());
         check(mm.second->config.id());
     }