CI: Adapt Homebrew Cask generation

[why] The Cask-fonts repo has been incorporated into the main Cask repo. On releases we would use an obsolete repository to create the Cask into.

The casks are now in a dedicated subdirectory (Casks/font) and under that we have one subdirectory for each font name starting character (to have a smaller number of files per directory I assume).

[how] Change the repository path.

Put part of the new (longer) path, the fixed part, just in the CI process, while the generator script takes care about the one-directory-per-startchar substructure.

Fixes: #1635

Requirements / Checklist

[x] Read the Contributing Guidelines
[ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx
[ ] If this contains a font/glyph add its origin as background info below (e.g. URL)
[ ] Verified the license of any newly added font, glyph, or glyph set. License is: xxx

What does this Pull Request (PR) do?

Update the Cask generation and workflow to the upstream (or is that downstream) architectural changes.

How should this be manually tested?

Let the workflow run and examine the resulting PR, then close the PR :-D Well, we need some change in our repo that causes the casks to differ or the PR will be empty.

ryanoasis / nerd-fonts