Fix: set correct 'currently executing' in fork_join task.

This is required to correctly execute multiple, consecutive wait_for_all calls in one fork_join_sub_task.

lib/pls/src/internal/scheduling/fork_join_task.cpp

@@ -21,14 +21,10 @@ fork_join_sub_task::fork_join_sub_task(const fork_join_sub_task &other) :
 
 void fork_join_sub_task::execute() {
   PROFILE_WORK_BLOCK("execute sub_task")
+  auto last_executing = tbb_task_->currently_executing_;
   tbb_task_->currently_executing_ = this;
-  if (executed) {
-    PLS_ERROR("Double Execution!")
-  }
-  executed = true;
-  executed_at = base::this_thread::state<thread_state>()->id_;
   execute_internal();
-  tbb_task_->currently_executing_ = nullptr;
+  tbb_task_->currently_executing_ = last_executing;
   PROFILE_END_BLOCK
   wait_for_all();