CI: Temporarily install CMake 3.x

Many vcpkg dependencies do not work with CMake 4.x, which GitHub Actions
now bundles by default. Install the latest 3.x from Kitware for now.

.github/actions/setup/action.yml

@@ -31,7 +31,7 @@ runs:
         sudo add-apt-repository 'deb http://apt.llvm.org/noble/ llvm-toolchain-noble-19 main'
 
         sudo apt-get update -y
-        sudo apt-get install -y autoconf autoconf-archive automake build-essential ccache clang-19 clang++-19 cmake curl fonts-liberation2 \
+        sudo apt-get install -y autoconf autoconf-archive automake build-essential ccache clang-19 clang++-19 curl fonts-liberation2 \
             gcc-13 g++-13 libegl1-mesa-dev libgl1-mesa-dev libpulse-dev libssl-dev \
             libstdc++-13-dev lld-19 nasm ninja-build qt6-base-dev qt6-tools-dev-tools tar unzip zip
 
@@ -61,6 +61,34 @@ runs:
         brew update
         brew install autoconf autoconf-archive automake bash ccache coreutils llvm@19 nasm ninja qt unzip wabt
 
+    # FIXME: GitHub Actions now bundles CMake 4.0, which breaks many vcpkg dependencies. See:
+    #        https://github.com/microsoft/vcpkg/issues/44726
+    - name: 'Install CMake 3.x'
+      shell: bash
+      run: |
+        set -e
+
+        if ${{ inputs.os == 'Linux' }} ; then
+          cmake_os="linux"
+          cmake_bin="bin"
+
+          if ${{ inputs.arch == 'x86_64' }} ; then
+            cmake_arch="x86_64"
+          elif ${{ inputs.arch == 'arm64' }} ; then
+            cmake_arch="aarch64"
+          fi
+        elif ${{ inputs.os == 'macOS' || inputs.os == 'Android' }} ; then
+          cmake_os="macos"
+          cmake_arch="universal"
+          cmake_bin="CMake.app/Contents/bin"
+        fi
+
+        wget https://github.com/Kitware/CMake/releases/download/v3.31.6/cmake-3.31.6-${cmake_os}-${cmake_arch}.tar.gz
+        tar -xzf ./cmake-3.31.6-${cmake_os}-${cmake_arch}.tar.gz
+        rm ./cmake-3.31.6-${cmake_os}-${cmake_arch}.tar.gz
+
+        echo "${{ github.workspace }}/cmake-3.31.6-${cmake_os}-${cmake_arch}/${cmake_bin}" >> $GITHUB_PATH
+
     - name: 'Set required environment variables'
       if: ${{ inputs.os == 'Linux' && inputs.arch == 'arm64' }}
       uses: actions/github-script@v7