[CI] fix bench_jit_per_token_group_quant_8bit sglang import cascade

[arathi-hlab]

Summary

Follow-up to #443. The Run Sglang Kernel Benchmarks step is now failing on bench_jit_per_token_group_quant_8bit.py with:

ModuleNotFoundError: No module named 'orjson'
  File ".../sgl_kernel/gemm.py", line 99, in sgl_per_token_group_quant_8bit
    from sglang.srt.utils import get_bool_env_var
  File "/root/sglang/python/sglang/__init__.py", line 21, in <module>
    ...
  File "/root/sglang/python/sglang/srt/utils/common.py", line 82, in <module>
    import orjson

Root cause

sgl_kernel.gemm.sgl_per_token_group_quant_8bit only imports sglang.srt.utils.get_bool_env_var when enable_v2 is None, to read the SGLANG_PER_TOKEN_GROUP_QUANT_8BIT_V2 env var. The bench was leaving enable_v2 unset, so PYTHONPATH=/root/sglang/python (added in #443 to unblock this) made sglang findable — but sglang/__init__.py cascades into srt/utils/common.py which pulls orjson, psutil, pybase64, requests, starlette, torchvision, … none installed in the CI container. Chasing those with pip installs is fragile.

Fix

• Pin enable_v2=False in the AOT calls in bench_jit_per_token_group_quant_8bit.py so sgl_kernel never reaches the sglang-lookup branch. v2 is already covered by bench_jit_per_token_group_quant_8bit_v2.py.
• Drop the export PYTHONPATH=/root/sglang/python from the bench step; nothing else in that step needs it.