Mark test_pthread_c11_threads_proxied as flaky (#22624)

Only the proxied version is flaky but all the variants need to be
marked.

Tracked in #22617

Author: dschuff

test/test_core.py

@@ -9102,6 +9102,7 @@ def test_pthread_create(self):
     self.set_setting('STRICT')
     self.do_run_in_out_file_test('core/pthread/create.c')
 
+  @flaky('https://github.com/emscripten-core/emscripten/issues/22617')
   @node_pthreads
   @parameterized({
     '': ([],),