ci(vllm): use bigger-runner instead of source build

The prebuilt vllm 0.14.1+cpu wheel requires SIMD instructions (AVX-512
VNNI/BF16) that stock ubuntu-latest GitHub runners don't support —
vllm.model_executor.models.registry SIGILLs on import during LoadModel.

Source compilation works but takes 30-40 minutes per CI run, which is
too slow for an e2e smoke test. Instead, switch tests-vllm-grpc to the
bigger-runner self-hosted label (already used by backend.yml for the
llama-cpp CUDA build) — that hardware has the required SIMD baseline
and the prebuilt wheel runs cleanly.

FROM_SOURCE=true is kept as an opt-in escape hatch:
- install.sh still has the CPU source-build path for hosts that need it
- backend/Dockerfile.python still declares the ARG + ENV
- Makefile docker-build-backend still forwards the build-arg when set
Default CI path uses the fast prebuilt wheel; source build can be
re-enabled by exporting FROM_SOURCE=true in the environment.

backend/Dockerfile.python

@@ -195,8 +195,9 @@ COPY backend/backend.proto /${BACKEND}/backend.proto
 COPY backend/python/common/ /${BACKEND}/common
 COPY scripts/build/package-gpu-libs.sh /package-gpu-libs.sh
 
-# Optional per-backend source build toggle (e.g. vllm on CPU needs to
-# compile against the host SIMD instead of using the prebuilt wheel).
+# Optional per-backend source build toggle (e.g. vllm on CPU can set
+# FROM_SOURCE=true to compile against the build host SIMD instead of
+# pulling a prebuilt wheel). Default empty — most backends ignore it.
 ARG FROM_SOURCE=""
 ENV FROM_SOURCE=${FROM_SOURCE}