CI: Bump vlc-debian-unstable image

This image contains a prebuilt protobuf-compiler

Note: the Debian job is expected to fail due to an issue with libtool & -lpthread

This was hidden by the fact that we were using the contrib version of sqlite, which was causing -lpthread to be added to the media library LDFLAGS through pkg-static.sh

This image includes a recent enough version, so we'll use the regular .pc which should cause the build to fail.

A later MR will provide a patch for libtool in order to fix the build