Comment 1 for bug 1975901

> auto-sync/ubuntu-bionic-18.04-amd64-server-20220523-disk1.img

This is a combination of:
1. charm's prefix ("auto-sync/")
2. pubname ("ubuntu-bionic-18.04-amd64-server-20220523")
3. item_name with "-" prefix if the pubname doesn't end with item_name ("-disk1.img)

2. & 3. are from simplestreams instead of the charm.

https://cloud-images.ubuntu.com/releases/streams/v1/com.ubuntu.cloud:released:download.json