Fix a multi-process collision that causes import failure

[xiaoman-pipelet]

This is for fixing the case that somehow there's another pysssss instance that tries to create this extension
I am not fully certain how did I end up running into below errors, i.e., starting two pysssss instances?
But it helps my case, and I think the change improves robustness anyways.

Traceback (most recent call last):
  File "/app/ComfyUI/nodes.py", line 2133, in load_custom_node
    module_spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/app/ComfyUI/custom_nodes/ComfyUI-Custom-Scripts/__init__.py", line 10, in <module>
    if init():
  File "/app/ComfyUI/custom_nodes/ComfyUI-Custom-Scripts/pysssss.py", line 172, in init
    install_js()
  File "/app/ComfyUI/custom_nodes/ComfyUI-Custom-Scripts/pysssss.py", line 130, in install_js
    dst_dir = get_web_ext_dir()
  File "/app/ComfyUI/custom_nodes/ComfyUI-Custom-Scripts/pysssss.py", line 62, in get_web_ext_dir
    os.makedirs(dir)
  File "/usr/lib/python3.10/os.py", line 225, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/app/ComfyUI/web/extensions/pysssss'