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.

.github/workflows/test-extra.yml

@@ -505,8 +505,12 @@ jobs:
   tests-vllm-grpc:
     needs: detect-changes
     if: needs.detect-changes.outputs.vllm == 'true' || needs.detect-changes.outputs.run-all == 'true'
-    runs-on: ubuntu-latest
-    timeout-minutes: 120
+    # The prebuilt vllm CPU wheel is compiled with AVX-512 VNNI/BF16
+    # instructions; stock ubuntu-latest runners SIGILL on import of
+    # vllm.model_executor.models.registry. bigger-runner has newer
+    # hardware that supports the required SIMD.
+    runs-on: bigger-runner
+    timeout-minutes: 90
     steps:
       - name: Clone
         uses: actions/checkout@v6
@@ -521,12 +525,6 @@ jobs:
           sudo rm -rf /usr/share/dotnet /opt/ghc /usr/local/lib/android /opt/hostedtoolcache/CodeQL || true
           df -h
       - name: Build vllm (cpu) backend image and run gRPC e2e tests
-        env:
-          # GitHub Actions runners don't all support the SIMD instructions
-          # the prebuilt vllm CPU wheel was compiled against (SIGILL in
-          # vllm.model_executor.models.registry on import). Build vllm from
-          # source so it targets the actual CI CPU.
-          FROM_SOURCE: "true"
         run: |
           make test-extra-backend-vllm
   tests-acestep-cpp: