Use full path for dconf in posttrans script
    
    We don't have PATH correctly set up to contain /app/bin when installing
    packages for flatpak containers. This led to the 'dconf update'
    posttrans script to fail to execute when building dconf-editor flatpak.
    
    As a workaround, just use the full path here.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1950829