ci/mac: install libplacebo from main branch

install libplacebo from main brach as a temporary measure till a new
version is tagged, to work around a major MoltenVK rendering
synchronisation problem.

Fixes mpv-player#17258

Author: Akemi

.github/workflows/build.yml

@@ -303,9 +303,11 @@ jobs:
             brew update
           fi
           if brew list ffmpeg &> /dev/null; then brew unlink ffmpeg; fi
+          if brew list libplacebo &> /dev/null; then brew unlink libplacebo; fi
           brew install -q autoconf automake pkgconf libtool python freetype fribidi little-cms2 \
-            luajit libass ffmpeg-full meson rust uchardet mujs libplacebo molten-vk vulkan-loader vulkan-headers \
+            luajit libass ffmpeg-full meson rust uchardet mujs molten-vk vulkan-loader vulkan-headers \
             libarchive libbluray libcaca libcdio-paranoia libdvdnav rubberband zimg
+          brew install --HEAD libplacebo
           brew link ffmpeg-full
 
       - name: Build with meson