Add TF_PYTHON_VERSION env var to set Python version in macOS arm64 CI
Now that [hermetic Python is enabled for TensorFlow](https://github.com/tensorflow/tensorflow/commit/e85860e8382a460a0dd8547a536e5eaaf9096a9f), we need to add the `TF_PYTHON_VERSION` to be able to set the Python version. This will also fix the currently [failing Python 3.9 and 3.11 macOS arm64 CI builds](https://tensorflow-ci.macstadium.com/job/tensorflow-as-build-nightly/).  

cc: @cjflan @kulinseth 