fix: deadlock in IO.Promise.resolve (#3693)

src/runtime/object.cpp

@@ -788,6 +788,7 @@ class task_manager {
     void resolve(lean_task_object * t, object * v) {
         unique_lock<mutex> lock(m_mutex);
         if (t->m_value) {
+            lock.unlock(); // `dec(v)` could lead to `deactivate_task` trying to take the lock
             dec(v);
             return;
         }