[MacPorts] #69661: Disable Homebrew on Apple Silicon GitHub Actions CI runners

[MacPorts]

#69661: Disable Homebrew on Apple Silicon GitHub Actions CI runners
----------------------------+---------------------
 Reporter:  ryandesign      |      Owner:  admin@…
     Type:  defect          |     Status:  new
 Priority:  Normal          |  Milestone:
Component:  server/hosting  |    Version:
 Keywords:                  |       Port:
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 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 being used, for example in the run for [https://github.com/macports
 /macports-ports/pull/23327 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 -`

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Ticket URL: <https://trac.macports.org/ticket/69661>
MacPorts <https://www.macports.org/>
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