fix: unit tests env will need clear too

[yihong0618]

Summary

if we have dify env file .env the unit tests will failed too, because the .env timeout is 600 and the tests env is 60
like

example_env_file = '/private/var/folders/c3/51t_3cqj7bx98dq1ptyy0v700000gn/T/pytest-of-hyi/pytest-28/test_dify_config0/.env'

    def test_dify_config(example_env_file):
        # load dotenv file with pydantic-settings
        config = DifyConfig(_env_file=example_env_file)
    
        # constant values
        assert config.COMMIT_SHA == ""
    
        # default values
        assert config.EDITION == "SELF_HOSTED"
        assert config.API_COMPRESSION_ENABLED is False
        assert config.SENTRY_TRACES_SAMPLE_RATE == 1.0
    
        # annotated field with default value
>       assert config.HTTP_REQUEST_MAX_READ_TIMEOUT == 60
E       AssertionError: assert 600 == 60
E        +  where 600 = DifyConfig(ENTERPRISE_ENABLED=False, CAN_REPLACE_LOGO=False, SENTRY_DSN='', SENTRY_TRACES_SAMPLE_RATE=1.0, SENTRY_PROF..., CONSOLE_CORS_ALLOW_ORIGINS=['http://127.0.0.1:3000', '*'], WEB_API_CORS_ALLOW_ORIGINS=['http://127.0.0.1:3000', '*']).HTTP_REQUEST_MAX_READ_TIMEOUT

/Users/hyi/prs/dify/api/tests/unit_tests/configs/test_dify_config.py:53: AssertionError

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods