Need env variable to avoid attempted download of nltk artifacts

[jamie256]

You can pre-download the vader lexicon but we need to also support not making external http calls.

Lets skip this line conditionally on env var: https://github.com/whylabs/langkit/blob/f4f2404d4d58b4c46ce003f054af700d730e534f/langkit/sentiment.py#L39

Alternatively, consider replacing this metric with vader_sentiment.