Install NLTK for edxapp workers

For ORA2 AI grading, we will need to make the NLTK corpus available to the edxapp workers.  NLTK finds the corpus using the environment variable `NLTK_DATA`, so this PR: (a) downloads the NLTK corpus from S3 (same as the ORA role) and (b) adds the `NLTK_DATA` to the edxapp worker environment.

@feanil I set this up for testing purposes, but if this approach seems reasonable then it would resolve [TKS-1835](https://edx-wiki.atlassian.net/browse/TKTS-1835)