Add end-to-end continuous integration tests for Homebrew distribution

[mattt]

As discussed in #196 and #197, we should make sure the swift-doc executable installed by Homebrew continues to work. Here's what I'm thinking:

• Create a new GitHub Actions workflow that runs on pushes to master as well as on a cron schedule (once per day, perhaps)
• In that action, use the test bundle from Add end-to-end tests for command-line interface #199 to work on swift-doc installed by Homebrew instead of the one built from source (this could be configured via a command argument or environment variable)
• Set up the workflow to test brew install swift-doc both building from source and downloading from a bottle, if available
• Add corresponding CI tests in @SwiftDocOrg/homebrew-formulae

[mattt]

Fixed by #243.