Install Doxygen cmake-multi-platform.yml

CMake failure due to missing Doxygen installation

.github/workflows/cmake-multi-platform.yml

@@ -89,6 +89,11 @@ jobs:
     #     echo "${{github.workspace}}/qt/Qt/${{ env.QT_VERSION }}/${{ env.MINGW_PATH }}/bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
     #     refreshenv
 
+    - name: Install Doxygen (Ubuntu)
+      run: |
+        sudo apt-get install doxygen
+      if: startsWith(matrix.config.os,'ubuntu-')
+
     - name: Set reusable strings
       # Turn repeated input strings (such as the build output directory) into step outputs. These step outputs can be used throughout the workflow file.
       id: strings