ci: Fix deployment of GCC < 14 under macOS

[amyspark]

Detailed description

@dragonmux just shared with me a macOS build log, and I noticed that it started to fail because Homebrew has started deploying GCC 14.1.

This PR ensures that Homebrew can cleanly deploy earlier versions by unlinking the version provided by GitHub Actions, before installing the requested GCC version.

Your checklist for this pull request

• [X] I've read the Code of Conduct
• [X] I've read the guidelines for contributing to this repository
• [X] It builds for hardware native (see Building the firmware)
• [X] It builds as BMDA (see Building the BMDA)
• [X] I've tested it to the best of my ability
• [X] My commit messages provide a useful short description of what the commits do

Closing issues