Use brew --prefix for PATH in build-mac-dmg.sh (Apple Silicon support)

[geekrebel]

installer/build-mac-dmg.sh currently starts with:

# XXX: These paths should be set using `brew prefix` commands,
#      for future-proofing against upgrades
PATH=/usr/local/Cellar/python@3.7/3.7.9_2/Frameworks/Python.framework/Versions/3.7/bin:/opt/local/bin:/opt/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/qt5/5.5/clang_64/bin:/opt/X11/bin

Two problems:

1. The /usr/local/Cellar/python@3.7/3.7.9_2/... path is wrong on any
   Apple Silicon Mac (Homebrew there lives at /opt/homebrew), so the script
   cannot run end-to-end on an arm64 runner. Python 3.7 itself has also been
   EOL since June 2023.
2. The /usr/local/qt5/5.5/clang_64/bin entry is similarly stale; Qt 5.5
   was released in 2015, and the rest of the project actually uses Qt 5.15
   (see freeze.py:477).

Neither of those entries is actually used by this script: a grep confirms
build-mac-dmg.sh never invokes python, qmake, or any Qt tool. The only
thing it needs on PATH is Homebrew's bin for appdmg; all of codesign,
xcrun, spctl, stapler, and the coreutils are under /usr/bin and
/bin already.

This PR replaces the hard-coded line with a small brew --prefix probe that:

• resolves to /opt/homebrew on Apple Silicon,
• resolves to /usr/local on Intel Macs (so existing Intel Mac build
  pipelines produce a byte-identical PATH, with no behavioural change),
• falls back to /usr/local if Homebrew is not installed at all.

It also resolves the existing # XXX: These paths should be set using brew prefix commands TODO comment in the file.

This is the third in a small series of Apple Silicon contributor PRs
(following #6003 "Apple VideoToolbox" label rename, and #6004 arch-aware
DMG naming). The next planned change is the /Users/jonathanthomas/...
hard-coded developer path in installer/fix_qt5_rpath.py:115.