JS_TEST fails on ARM instances on node.js binaries installed for wrong CPU architecture

JS_TESTs can install the required node.js binaries on the fly before running the test steps. Unfortunately the installation steps hardcode the CPU architecture as x86_64 in https://github.com/apache/impala/blob/master/tests/run-js-tests.sh#L27, which causes a failure later if the tests are run on an instance with an aarch64 (Graviton) CPU.