Flaky macOS CI

[Alasdair]

Macos CI has started failing occasionally with

==> Pouring pkgconf--2.3.0_1.arm64_sonoma.bottle.tar.gz
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /opt/homebrew
Could not symlink bin/pkg-config
Target /opt/homebrew/bin/pkg-config
is a symlink belonging to [email protected]. You can unlink it:
  brew unlink [email protected]

To force the link and overwrite all conflicting files:
  brew link --overwrite pkgconf

To list all files that would be deleted:
  brew link --overwrite pkgconf --dry-run

Possible conflicting files are:
/opt/homebrew/bin/pkg-config -> /opt/homebrew/Cellar/[email protected]/0.29.2_3/bin/pkg-config
/opt/homebrew/share/aclocal/pkg.m4 -> /opt/homebrew/Cellar/[email protected]/0.29.2_3/share/aclocal/pkg.m4
/opt/homebrew/share/man/man1/pkg-config.1 -> /opt/homebrew/Cellar/[email protected]/0.29.2_3/share/man/man1/pkg-config.1

but this is not consistent, sometimes this step succeeds and it fails later with

The following system packages will first need to be installed:
    pkgconf

<><> Handling external dependencies <><><><><><><><><><><><><><><><><><><><>  🐫 

opam believes some required external dependencies are missing. opam can:

(even though pkg-config is installed)

Other times the CI job succeeds. Attempting to fix one issue seems to cause breakage in the other cases.

[jordancarlin]

Seems likely this was related to actions/runner-images#10984. Sounds like it should be resolved now.