ci(macos): Fix brew upgrade failure, bis

[erikmd]

• Kind: bugfix

Description

cf. 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 documentation

Note to maintainers

• Read this wiki page
• Make sure the PR has a milestone
• Assign yourself before merging
• We can squash-merge 1-commit PRs (use a header with a conventional-commit type, add a footer with Fix #… if need be)

[erikmd]

To be merged before #516