Switch to ffmpeg5, move install before devel_qt

Currently, devel_qt installs a stripped version of ffmpeg5 (ffmpeg-5-mini-libs) that contains no codecs, and ffmpegthumbs installs the ffmpeg4 devel packages. This is a problem for Kdenlive testing because MLT's melt binary links against ffmpeg5 and cannot do any encoding, which is something we want to use in our tests. My proposal switches ffmpegthumbs to use ffmpeg5 (I checked that ffmpegthumbs still compiled with it on Qt 5.15). Moving the dependency above the devel_qt5 dependency ensures the full ffmpeg devel packages are installed instead of ffmpeg-5-mini-libs without causing package conflict.