Fix: add --no-build-isolation to pip wheel for flash-attention

Without this flag, pip runs the build in an isolated environment
that doesn't have access to torch in the venv.

vllm/Dockerfile

@@ -122,7 +122,7 @@ RUN apt-get update && apt-get install -y build-essential cmake gcc && \
     FLASH_ATTENTION_FORCE_BUILD="TRUE" \
     FLASH_ATTENTION_FORCE_CXX11_ABI="FALSE" \
     FLASH_ATTENTION_SKIP_CUDA_BUILD="FALSE" \
-    pip wheel . -v --no-deps -w ./wheels/ && \
+    pip wheel . -v --no-deps --no-build-isolation -w ./wheels/ && \
     cp wheels/*.whl /wheels/
 
 FROM build-base AS build-vllm