vllm: resolve the build requirements file via {package}

cibuildwheel's before-build cwd is not guaranteed to be the package
directory when package-dir points at a subdirectory, so anchor the
requirements path explicitly.

Author: claude

.github/workflows/build-vllm.yml

@@ -93,7 +93,7 @@ jobs:
             | sh -s -- -y --default-toolchain none
           CIBW_BUILD_FRONTEND: "pip; args: --no-build-isolation"
           CIBW_BEFORE_BUILD: >-
-            pip install --only-binary=:all: -r requirements/build/cpu.txt
+            pip install --only-binary=:all: -r {package}/requirements/build/cpu.txt
           CIBW_ENVIRONMENT: >-
             VLLM_TARGET_DEVICE=cpu
             VLLM_VERSION_OVERRIDE=${{ env.VLLM_VERSION }}