sycl : reduce the build threads to half (#30236)

.devops/intel.Dockerfile

@@ -50,7 +50,7 @@ RUN if [ "${GGML_SYCL_F16}" = "ON" ]; then \
     fi && \
     echo "Building with dynamic libs" && \
     cmake -B build -DGGML_NATIVE=OFF -DGGML_SYCL=ON -DCMAKE_C_COMPILER=icx -DCMAKE_CXX_COMPILER=icpx -DGGML_BACKEND_DL=ON -DGGML_CPU_ALL_VARIANTS=ON -DLLAMA_BUILD_TESTS=OFF ${OPT_SYCL_F16} && \
-    cmake --build build --config Release -j$(nproc)
+    cmake --build build --config Release -j$((($(nproc)+1)/2))
 
 RUN mkdir -p /app/lib && \
     find build -name "*.so*" -exec cp -P {} /app/lib \;