fix nltk_data env variable usage

Author: Guillaume De Saint Martin

build_scripts/windows.ps1

@@ -6,7 +6,7 @@ python scripts/python_file_lister.py bin/octobot_packages_files.txt $env:OCTOBOT
 python scripts/insert_imports.py $env:OCTOBOT_REPOSITORY_DIR/octobot/cli.py
 Copy-Item bin $env:OCTOBOT_REPOSITORY_DIR -recurse
 cd $env:OCTOBOT_REPOSITORY_DIR
-python ../scripts/fetch_nltk_data.py words $NLTK_DATA
+python ../scripts/fetch_nltk_data.py words $env:NLTK_DATA
 python setup.py build_ext --inplace
 python -m PyInstaller bin/start.spec
 Rename-Item dist/OctoBot.exe OctoBot_windows.exe