- I was going through the `sorbet/files.yaml` moving various things to
  `true`, playing around locally, and happened upon some files that it
  was tracking that had been deleted.
- We want to do some automation of new files, but I'm not sure we'd ever
  considered the possibility of code being removed. :-D
- This adds a `--prune-files-list` switch and updates the Tapioca GitHub
  Actions workflow to use it. Any changes to the `sorbet/files.yaml`
  file will be committed as part of the scheduled Tapioca update job.

Once every 24h should be sufficient for both of these.

This avoids weirdness like the merged PR commits being pushed to scheduled opened PRs after merging.

It also avoids wasting resources.

- Ensure `spdx.yml` has same fixes as `tapioca.yml`.
- Don't `reset --hard` to ensure that `brew` commands used are those in
  the same branch rather than always from `origin/master`
- Don't check for an existing branch but instead force-push to it if
  there's any updates.
- Provide sample `brew typecheck` command that can be used.

- In all of the PRs that this Action triggered, the `tapioca-update`
  branch was >200 commits behind the latest `master`.
- As per https://github.com/Homebrew/brew/pull/8435#issuecomment-678546024,
  we shouldn't rely on the `Homebrew/actions/setup-homebrew` master
  (the `origin/master` ref) branch being up to date, so resetting to
  `origin/master` was a bad idea.
- A side effect of this is that when we raise PRs to change the Action,
  the PRs that it generates will include local commits. But functionally
  all we care about is the Action that runs on a schedule.

- ensure we reset to `origin/master` so we only have the commit we care
  about in the PR
- ensure we `git add` to include any new files
- don't use `git commit -a` and instead rely only on files added with
  `git add`