Stop using Homebrew to install chezmoi

[felipecrs]

The thing is that Homebrew takes too long to install in first time, because it downloads so many dependencies that I don't think they are actually that much required.

But since we can't rely on the chezmoi upgrade to self-upgrade its binary because it calls the GitHub API, the way to go is to wrap a function for upgrading chezmoi in the full-upgrade command.