fix Ray runtime env no working dir issue

[kunling-anyscale]

What does this PR do?

Respect Ray Job runtime_env in PPO runs by fixing get_ppo_ray_runtime_env():

• Do not inject working_dir=None (omit key entirely if absent).
• Pass through working_dir from RAY_JOB_CONFIG_JSON_ENV_VAR when provided.
• Avoid overriding env vars already present in os.environ.

This prevents VERL from clobbering a valid job working_dir and stabilizes env-var merge semantics when users submit via Ray Jobs.

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[wuxibin89]

@kunling-anyscale Please format code with: https://github.com/volcengine/verl/blob/main/CONTRIBUTING.md#code-linting-and-formatting

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