Concurrent mode start failed with `uvicorn`

[Ox0400]

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Bug summary

Process SpawnProcess-7:
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/usr/local/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.10/site-packages/uvicorn/_subprocess.py", line 76, in subprocess_started
    target(sockets=sockets)
  File "/usr/local/lib/python3.10/site-packages/uvicorn/server.py", line 61, in run
    return asyncio.run(self.serve(sockets=sockets))
  File "/usr/local/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/local/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.10/site-packages/uvicorn/server.py", line 68, in serve
    config.load()
  File "/usr/local/lib/python3.10/site-packages/uvicorn/config.py", line 479, in load
    self.loaded_app = self.loaded_app()
  File "/usr/local/lib/python3.10/site-packages/prefect/server/api/server.py", line 667, in create_app
    ui_app = create_ui_app(ephemeral)
  File "/usr/local/lib/python3.10/site-packages/prefect/server/api/server.py", line 408, in create_ui_app
    create_ui_static_subpath()
  File "/usr/local/lib/python3.10/site-packages/prefect/server/api/server.py", line 385, in create_ui_static_subpath
    copy_directory(prefect.__ui_static_path__, static_dir)
  File "/usr/local/lib/python3.10/site-packages/prefect/server/api/server.py", line 228, in copy_directory
    shutil.rmtree(destination)
  File "/usr/local/lib/python3.10/shutil.py", line 731, in rmtree
    onerror(os.rmdir, path, sys.exc_info())
  File "/usr/local/lib/python3.10/shutil.py", line 729, in rmtree
    os.rmdir(path)
OSError: [Errno 39] Directory not empty: '/usr/local/lib/python3.10/site-packages/prefect/server/ui_build/assets'

Reproduction

export WEB_CONCURRENCY=10
prefect server start
``

Error

No response

Versions

2.18.0

Additional context

No response