CI mac: enforce use of Qt5 even if Qt6 present

fixes failed run
<https://github.com/CESNET/UltraGrid/actions/runs/5925739298/job/16065753514>

From some point, it seems that Qt6 (Homebrew port _qt_) is already
installed but it doesn't bundle successfully. So enforce using Qt5
(previous symlink command didn't replace the `/usr/local/opt/qt` symlink
if already present).

\+ copy the link instead of linking it to prevent:

    Error: /usr/local/opt/qt@5 is not a valid keg

.github/scripts/macOS/prepare.sh

@@ -26,7 +26,8 @@ brew install qt@5
 brew install glm
 curl -L https://raw.githubusercontent.com/miniupnp/libnatpmp/master/natpmp_declspec.h -o /usr/local/include/natpmp_declspec.h
 
-sudo ln -s /usr/local/opt/qt@5 /usr/local/opt/qt
+rm -f /usr/local/opt/qt
+cp -af /usr/local/opt/qt@5 /usr/local/opt/qt
 
 .github/scripts/macOS/install_dylibbundler_v2.sh