Fix CI: add qtmultimedia to optional cmake prefix loop

Qt6TextToSpeech depends on Qt6Multimedia internally. Removing qtmultimedia
from the required-check loop also removed it from CMAKE_PREFIX_PATH, so
CMake could not find Qt6Multimedia when resolving Qt6TextToSpeech.

Move it to the optional loop — it is always present as a transitive dep
of qtspeech so it will always be picked up, without making it a hard
install requirement.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: KorigamiK

build_mac.sh

@@ -27,7 +27,7 @@ for formula in qtbase qtdeclarative qtsvg qtspeech mupdf; do
   cmake_prefixes+=("$prefix")
 done
 
-for formula in qtquicktimeline qtquick3d qtshadertools; do
+for formula in qtmultimedia qtquicktimeline qtquick3d qtshadertools; do
   prefix=$(brew --prefix "$formula" 2>/dev/null || true)
   if [[ -n $prefix && -d $prefix ]]; then
     cmake_prefixes+=("$prefix")