re-enable `--sync-light-client` in CI testnets (#4469)

libp2p issues related to operation cancellations have been addressed in
https://github.com/status-im/nim-libp2p/pull/816
This means we can once more enable `--sync-light-client` in CI, without
having to deal with spurious CI failures due to the cancellation issues.

scripts/launch_local_testnet.sh

@@ -1003,6 +1003,7 @@ for NUM_NODE in $(seq 0 $(( NUM_NODES - 1 ))); do
     --keymanager-token-file="${DATA_DIR}/keymanager-token" \
     --rest-port="$(( BASE_REST_PORT + NUM_NODE ))" \
     --metrics-port="$(( BASE_METRICS_PORT + NUM_NODE ))" \
+    --sync-light-client=on \
     ${EXTRA_ARGS} \
     &> "${DATA_DIR}/log${NUM_NODE}.txt" &