Implement zlint-autopull with Github Actions

[cpu]

Presently the automation that invokes the zlint-gtld-update binary periodically and creates PRs with the resulting diff lives in a separate zlint-autopull repo: https://github.com/cpu/zlint-autopull It's also based on Travis, where cron builds were limited to at most once a day.

This feels like something that could be converted to a Github Action. Potentially even removing the tld-update-bot's access to this repository.