the NUMA implementation of `static_thread_pool` leaks an object at shutdown

#1369 "fixed" a static-initialization-order-fiasco bug in `static_thread_pool` teardown by constructing a global `std::vector` and never destroying it. if stdexec is used in a `.so` that is repeatedly loaded and unloaded with `dlopen`/`dlclose`, it will leak an unbounded amount of memory. 

find a better solution.