Add autoupdate workflow

[haydenbaker]

Description of changes:

• Refactor and move the existing homebrew update workflow to this repo
• GH doesn't have a seamless way of triggering actions in a repo from another, so we'll have to use a token to do so (like we already do)
• This just simplifies the logic, separates the concern of updating the tap from the main repo, and puts it here (where it fits better)
• Also means that we can trigger a tap rollback if needed (by hand)

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