Set HF_SCRIPTS_VERSION to main
After renaming "master" to "main", the CI fails with
```
AssertionError: 'https://raw.githubusercontent.com/huggingface/datasets/main/datasets/_dummy/_dummy.py' not found in "Couldn't find a dataset script at /home/circleci/datasets/_dummy/_dummy.py or any data file in the same directory. Couldn't find '_dummy' on the Hugging Face Hub either: FileNotFoundError: Couldn't find file at https://raw.githubusercontent.com/huggingface/datasets/master/datasets/_dummy/_dummy.py"
```

This is because in the CI we were still using `HF_SCRIPTS_VERSION=master`. I changed it to "main"