Disable Homebrew on Apple Silicon GitHub Actions CI runners

[ryandesign (Ryan Carsten Schmidt)]

source:macports-ports/.github/workflows/bootstrap.sh​ has code that disables the pre-installed Homebrew in /usr/local. But we now have the macOS 14 runner which is on Apple Silicon and on Apple Silicon Homebrew doesn't install in /usr/local; it installs in /opt/homebrew. We haven't disabled that. So Homebrew is still present for our macOS 14 CI builds and it is being used, for example in the run for ​this PR:

DEBUG: system: cd "/opt/local/var/macports/build/_Users_runner_work_macports-ports_macports-ports_ports_textproc_po4a/po4a/work" && /usr/bin/gzip -dc '/opt/local/var/macports/distfiles/po4a/po4a-0.71.tar.gz' | /opt/homebrew/bin/gtar --no-same-owner -xf -