fix: continue to use flashinfer 0.1.6 temporarily

[zhyncs]

Motivation

If using the latest 0.2.0, CI cannot pass. Temporarily fix it to 0.1.6 and wait for FlashInfer to release 0.2.0.post1 or 0.2.1 before lifting the restriction again.

ref https://github.com/sgl-project/sglang/actions/runs/12407074179

cc @yzh119 @merrymercy

Modifications

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.