fix(vllm): build from source on CI to avoid SIGILL on prebuilt wheel

The prebuilt vllm 0.14.1+cpu wheel from GitHub releases is compiled with
SIMD instructions (AVX-512 VNNI/BF16 or AMX-BF16) that not every CPU
supports. GitHub Actions ubuntu-latest runners SIGILL when vllm spawns
the model_executor.models.registry subprocess for introspection, so
LoadModel never reaches the actual inference path.

- install.sh: when FROM_SOURCE=true on a CPU build, temporarily hide
  requirements-cpu-after.txt so installRequirements installs the base
  deps + torch CPU without pulling the prebuilt wheel, then clone vllm
  and compile it with VLLM_TARGET_DEVICE=cpu. The resulting binaries
  target the host's actual CPU.
- backend/Dockerfile.python: accept a FROM_SOURCE build-arg and expose
  it as an ENV so install.sh sees it during `make`.
- Makefile docker-build-backend: forward FROM_SOURCE as --build-arg
  when set, so backends that need source builds can opt in.
- Makefile test-extra-backend-vllm: call docker-build-vllm via a
  recursive $(MAKE) invocation so FROM_SOURCE flows through.
- .github/workflows/test-extra.yml: set FROM_SOURCE=true on the
  tests-vllm-grpc job. Slower but reliable — the prebuilt wheel only
  works on hosts that share the build-time SIMD baseline.

Answers 'did you test locally?': yes, end-to-end on my local machine
with the prebuilt wheel (CPU supports AVX-512 VNNI). The CI runner CPU
gap was not covered locally — this commit plugs that gap.

backend/Dockerfile.python

@@ -195,6 +195,11 @@ 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).
+ARG FROM_SOURCE=""
+ENV FROM_SOURCE=${FROM_SOURCE}
+
 RUN cd /${BACKEND} && PORTABLE_PYTHON=true make
 
 # Package GPU libraries into the backend's lib directory