cmake, ci: Build bundled FFmpeg with yuzu

Drops usage of CMAKE_DEPENDENT_OPTION to allow using
YUZU_USE_BUNDLED_FFMPEG as an option on any platform. CI then now builds
FFmpeg always, netting about 10 MB less used on the AppImage.

Also somewhat fixes YUZU_USE_BUNDLED_QT so that it can be used even if
CMake doesn't clean up its state after running the first find_package.