Automate `lake update` PRs

[chenson2018]

Following #293, we would like a workflow that automates running lake update. What I would like ideally is:

• at some cron interval (day, week?) the bot runs lake update and opens a PR
• if CI is a success, the bot places it on the merge queue
• on failure, the PR remains open so a maintainer can fix

This will need to be a little different from the Mathlib equivalent, as the bot will need to actually merge the PR versus leaving a bors comment.