Fix macOS 15 test runner by unlinking old openssl binary - #32448

PR summary

Our "Python 3.12 on macos-15" test runner has been failing all day with:

Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /opt/homebrew
Could not symlink bin/openssl
Target /opt/homebrew/bin/openssl
is a symlink belonging to openssl@1.1. You can unlink it:
  brew unlink openssl@1.1

To force the link and overwrite all conflicting files:
  brew link --overwrite openssl@3

To list all files that would be deleted:
  brew link --overwrite openssl@3 --dry-run

Possible conflicting files are:
/opt/homebrew/bin/openssl -> /opt/homebrew/opt/openssl@1.1/bin/openssl

brew unlink openssl@1.1 doesn't actually work, as the runner image (or Homebrew?) appears to make a manual link from "/opt/homebrew/bin/openssl" to the openssl@1.1 binary.

I searched GitHub for similar reports and took the solution found in y-scope/clp#2545.

AI Disclosure

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PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• [/] New and changed code is tested
• [/] Plotting related features are demonstrated in an example
• [/] New features and API changes have release notes
• [/] Documentation complies with general and docstring guidelines