ci(macos): Fix brew upgrade failure, ter

[erikmd]

• Kind: bugfix

Description

https://github.com/ocaml-sf/learn-ocaml/wiki/(CI)(macOS)-How-to-fix-spurious-brew-link-failures

Checklist

• Read the CONTRIBUTING.md guide and:
  • Use Atomic Commits so each commit gathers a single logical change
  • Use Conventional Commits regarding commit messages (needed by our release toolchain)
• Add/update tests
• Add/update documentation
• …

Note to maintainers

• Read this wiki page.
• Make sure the PR has a milestone.
• Assign yourself before merging.
• Either do a regular merge:
  • for PRs containing several commits following conventional-commits,
  • or for PRs containing 1 commit shared with a later PR (to preserve the SHA1)
• Or do a squash-merge:
  • for PRs containing only 1 commit (not shared with a later PR),
  • or for PRs containing several commits that need not be kept in the history;
  • → Update the commit message header with a conventional-commit type,
  • → Add a footer Close #… if a related issue exists.

[erikmd]

The CI looks good now, let's merge this tiny PR.