[easy] Allow user to set ANDROID_NDK in build_llama_android script (#12662)

The way it was written required hand-hacking to build locally. Now it
respects ANDROID_NDK from the environment.

Author: swolchok

.ci/scripts/build_llama_android.sh

@@ -19,7 +19,7 @@ install_executorch_and_backend_lib() {
   echo "Installing executorch and xnnpack backend"
   clean_executorch_install_folders
   mkdir cmake-android-out
-  ANDROID_NDK=/opt/ndk
+  ANDROID_NDK=${ANDROID_NDK:-/opt/ndk}
   BUCK2=buck2
   ANDROID_ABI=arm64-v8a
   cmake --preset llm \