add all to one script with deps/build_install/install_cached actions
- yml - use action @main
- cache: do not depend .github/scripts/Linux/prepare.sh, just the
catch-all SDL install file
- build SDL,SDL_ttf without entering the dir (as fluidsynth already does)
In U22.04 (current Linux CI image), fluidsynth depends on sdl2, which
is undesirable. Actually we need to avoid sdl2 linking altogether.
Build from sources alongside with SDL. This makes sense since SDL_mixer
used to be build there and fluidsynth is replacing it.
+ prevent libsdl2-2.0-0 from (accidentally) installing
SDL2 now obviously works on U20.04, because we are building there. It
doesn't work on U22.04, but neither the build with SDL 2.0.20 nor 2.26.2
does.
The problem seems actually to be the inclusion of Wayland libraries in
AppImage.
