python3Packages.langchain-community: disable flaky test (#408838)

pkgs/development/python-modules/langchain-community/default.nix

···
   disabledTests = [
     # requires bs4, aka BeautifulSoup
     "test_importable_all"
+    # flaky
+    "test_llm_caching_async"
   ];
 
   disabledTestPaths = [