Free default_rand_key after freeing Ractors

Ractor's free iterates through its TLS keys so we need to keep this
memory available until after Ractors are freed.

Minimal reproduction:

    RUBY_FREE_AT_EXIT=1 ./miniruby -e rand

vm.c

@@ -2994,7 +2994,6 @@ ruby_vm_destruct(rb_vm_t *vm)
         rb_thread_t *th = vm->ractor.main_thread;
         VALUE *stack = th->ec->vm_stack;
         if (rb_free_at_exit) {
-            rb_free_default_rand_key();
             rb_free_encoded_insn_data();
             rb_free_global_enc_table();
             rb_free_loaded_builtin_table();
@@ -3056,6 +3055,7 @@ ruby_vm_destruct(rb_vm_t *vm)
             if (rb_free_at_exit) {
                 rb_objspace_free_objects(objspace);
                 rb_free_generic_iv_tbl_();
+                rb_free_default_rand_key();
                 if (th) {
                     xfree(stack);
                     ruby_mimfree(th);