Add EXECUTORCH_BUILD_CUSTOM=ON for llama runner

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Author: larryliu0820

.ci/scripts/test_llama.sh

@@ -59,6 +59,7 @@ cmake_install_executorch_libraries() {
     retry cmake -DBUCK2="$BUCK" \
         -DCMAKE_INSTALL_PREFIX=cmake-out \
         -DCMAKE_BUILD_TYPE=Release \
+        -DEXECUTORCH_BUILD_CUSTOM=ON \
         -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
         -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
         -DEXECUTORCH_BUILD_OPTIMIZED=ON \