[ci] Add logic to change model to S3 path only when S3 CI env var is on

[khluu]

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[hmellor]

LGTM

[youkaichao]

why are we changing it in the test? I think we should update the served_model_name directly in

vllm/vllm/engine/arg_utils.py

Line 1150 in cbae7af

self.model = f"{MODEL_WEIGHTS_S3_BUCKET}/{self.model}"

?

[khluu]

why are we changing it in the test? I think we should update the served_model_name directly in

vllm/vllm/engine/arg_utils.py

Line 1150 in cbae7af

self.model = f"{MODEL_WEIGHTS_S3_BUCKET}/{self.model}"

?

Isn't the test not asserting value against served_model_name? I thought it was fixed in the params