CI mac: disable pkg-config and use pkgconf

pkgconf is more modern version and the CI on macos-14 now fails
because both pkg-config and pkgconf (presumably as a dependency) are to
be installed.

The erroa (the beginning, additional info omittedr):
```
==> Pouring pkgconf--2.3.0_1.arm64_sonoma.bottle.tar.gz
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /opt/homebrew
Could not symlink bin/pkg-config
Target /opt/homebrew/bin/pkg-config
is a symlink belonging to [email protected]. You can unlink it:
  brew unlink [email protected]
```

Both pkgconf and pkg-config can be installed automatically as a dependency
of some other packages so ensure a defined state (both installed,
pkg-config disabled).

.github/scripts/macOS/prepare.sh

@@ -31,6 +31,15 @@ echo "PKG_CONFIG_PATH=/usr/local/lib/pkgconfig" >> "$GITHUB_ENV"
 echo "/usr/local/opt/qt/bin" >> "$GITHUB_PATH"
 echo "DYLIBBUNDLER_FLAGS=$DYLIBBUNDLER_FLAGS" >> "$GITHUB_ENV"
 
+# Ensure that pkg-config is installed but unlinked.
+# This is to prevent interference with pkgconf - both can be installed as
+# a dependency of other packages so ensure a defined state (both installed;
+# pkgconf installed and enabled later).
+if ! brew list pkg-config 2>/dev/null; then
+        brew install pkg-config
+fi
+brew unlink pkg-config
+
 set -- \
         asciidoctor \
         autoconf \
@@ -46,7 +55,7 @@ set -- \
         molten-vk \
         opencv \
         ossp-uuid `#for cineform` \
-        pkg-config \
+        pkgconf \
         portaudio \
         qt \
         sdl2 \