[chore] Exclude nixl-cu13 from packages

[SumanthRH]

What does this PR do?

Excludes nixl cu13 from the packages.

After #1601 , vllm model resolution fails with

, ip=10.0.53.33, actor_id=4bb0516283686471f260700b04000000, repr=<skyrl.backends.skyrl_train.inference_servers.vllm_server_actor.VLLMServerActor object at 0x706173dea0f0>)
  File "/home/ray/anaconda3/lib/python3.12/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/home/ray/anaconda3/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/ray/session_2026-06-07_06-36-06_373207_3903/runtime_resources/working_dir_files/s3_anyscale-production-data-cld-hxkifz7xa22mwicp21nzkds1lw_org_xc6lv84h3d7m9dljcc17esfw2i_cld_hxkifz7xa22mwicp21nzkds1lw_runtime_env_packages_pkg_78d2d462c60fbf0f44597ff416ba6c12/skyrl/backends/skyrl_train/inference_servers/vllm_server_actor.py", line 279, in start
    await self._wait_until_healthy()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/ray/session_2026-06-07_06-36-06_373207_3903/runtime_resources/working_dir_files/s3_anyscale-production-data-cld-hxkifz7xa22mwicp21nzkds1lw_org_xc6lv84h3d7m9dljcc17esfw2i_cld_hxkifz7xa22mwicp21nzkds1lw_runtime_env_packages_pkg_78d2d462c60fbf0f44597ff416ba6c12/skyrl/backends/skyrl_train/inference_servers/vllm_server_actor.py", line 294, in _wait_until_healthy
    raise exc
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/ray/session_2026-06-07_06-36-06_373207_3903/runtime_resources/working_dir_files/s3_anyscale-production-data-cld-hxkifz7xa22mwicp21nzkds1lw_org_xc6lv84h3d7m9dljcc17esfw2i_cld_hxkifz7xa22mwicp21nzkds1lw_runtime_env_packages_pkg_78d2d462c60fbf0f44597ff416ba6c12/skyrl/backends/skyrl_train/inference_servers/vllm_server_actor.py", line 335, in _run_server
    self._engine = AsyncLLMEngine.from_engine_args(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ray/.cache/uv/builds-v0/.tmpHNSj9v/lib/python3.12/site-packages/vllm/v1/engine/async_llm.py", line 242, in from_engine_args
    vllm_config = engine_args.create_engine_config(usage_context)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ray/.cache/uv/builds-v0/.tmpHNSj9v/lib/python3.12/site-packages/vllm/engine/arg_utils.py", line 1627, in create_engine_config
    model_config = self.create_model_config()
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ray/.cache/uv/builds-v0/.tmpHNSj9v/lib/python3.12/site-packages/vllm/engine/arg_utils.py", line 1475, in create_model_config
    return ModelConfig(
           ^^^^^^^^^^^^
  File "/home/ray/.cache/uv/builds-v0/.tmpHNSj9v/lib/python3.12/site-packages/pydantic/_internal/_dataclasses.py", line 121, in __init__
    s.__pydantic_validator__.validate_python(ArgsKwargs(args, kwargs), self_instance=s)
pydantic_core._pydantic_core.ValidationError: 1 validation error for ModelConfig
  Value error, Model architectures ['Qwen3MoeForCausalLM'] failed to be inspected. Please check the logs for more details. [type=value_error, input_value=ArgsKwargs((), {'model': ...nderer_num_workers': 1}), input_type=ArgsKwargs]
    For further information visit https://errors.pydantic.dev/2.13/v/value_error

The root cause is the nixl package. The nixl package on PyPI is a meta package that includes both nixl-cu12 and nixl-cu13. nixl-cu13 ships a binary that breaks vllm model inspection.

I haven't figured out why the vllm-router upgrade triggered this, but the fix seems to work

[gemini-code-assist]

Code Review

This pull request updates pyproject.toml to add an override dependency for nixl-cu13 with the environment marker sys_platform == 'never'. This change prevents the installation of nixl-cu13 on Linux, resolving a collision with nixl-cu12 that breaks vLLM model inspection in CUDA-12 environments. There are no review comments, so I have no feedback to provide.

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[SumanthRH]

Megatron Models GPU CI passes: https://github.com/NovaSky-AI/SkyRL/actions/runs/27103746484/job/79989027090?pr=1756

The same workflow fails on main with the vllm model validation error: https://github.com/NovaSky-AI/SkyRL/actions/runs/27085057237/job/79937793420