vlc-snap: update to core24, kde-qt6-sdk and rockcraft
this MR uses snapcraft rocks to provide a snapcraft building environment in Docker (so without needing to rely on LXD)
it also install kde/qt base snap packages as theses are expected to be present in "destructive mode" (used by snapcraft in this configuration)
VLC dependencies packages as pre-installed to avoid re-downloading every system dependencies form the package each time snapcraft package is built