Fix flatpak appstream metadata assembling
    
    Adapt to upstream changes in solenv/bin/assemble-flatpak-appdata*
    scripts: Separate step1 and step2 are gone and there is only a single
    script to generate flatpak appstream metadata now.
    
    See https://github.com/LibreOffice/core/commit/48d09aa85e01d881995c660f845b60bd8088e7c0