[Release Only] Fix llama-runner jobs on ARM linux (#14677)

The libcxx version is too old on the ARM linux runners. Install a newer
version with Conda as a band-aid fix. See
#14679 for details and to
track the long-term fix.

Note that this is intentionally done on the release branch, as the issue
only affects the jobs using the rc wheels. We'll want to solve it more
generally, but the fix needs to be tested here. See the tracking task.

Author: GregoryComer

.ci/scripts/setup-linux.sh

@@ -23,6 +23,12 @@ else
 fi
 build_executorch_runner "${BUILD_TOOL}" "${BUILD_MODE}"
 
+# Fix for libcxx version issues with PyTorch prebuilts.
+# Tracking in https://github.com/pytorch/executorch/issues/14679.
+if [ "$(uname -m)" == "aarch64" ]; then
+  conda install -y -c conda-forge libstdcxx-ng
+fi
+
 if [[ "${GITHUB_BASE_REF:-}" == *main* || "${GITHUB_BASE_REF:-}" == *gh* ]]; then
   do_not_use_nightly_on_ci
 fi