images: Download the multimedia demopack with wget

The downloader in Debos using the Golang stdlib often hits a TLS timeout on the Jenkins workers for causes still not clearly understood:

Download started: 'https://images.apertis.org/media/multimedia-demo.tar.gz' -> '/scratch/multimedia-demo.tar.gz'
Action `download` failed at stage Run, error: Get https://images.apertis.org/media/multimedia-demo.tar.gz: net/http: TLS handshake timeout

As a first step to investigate the issue, download the multimedia demopack with wget in debug mode.