Add missing `servingEngineSpec.env`

[arno4000]

According to https://github.com/vllm-project/production-stack/blob/main/tutorials/assets/values-12-otel-vllm.yaml, it should be possible to specify a env block for the whole servingEngineSpec. But in the values.yaml file, only servingEngineSpec.modelSpec[].env is supported. I think it would be nice if the environment variables could be defined for the whole servingEngineSpec, currently I have to set the proxy for each modelSpec.