macOS: Add /opt/homebrew to CMAKE_SYSTEM_PREFIX_PATH

Brad Kingrequested to merge
brad.king/cmake:macos-homebrew-apple-silicon into master

According to https://brew.sh/2020/12/01/homebrew-2.6.0/ the /opt/homebrew directory is recommended for installing ARM architecture brew packages.

Fixes: #21585 (closed)
Backport: release

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