Fix upload destination

The upload.sh script uses etc/os-release to figure out what is the current Apertis release channel (for instance, v2021) and computes the upload destination accordingly.

Unfortunately it is currently set up to run in a Ubuntu 16.06 Xenial container, which means that uploads end up in places like /srv/images/public/daily/xenial/20201215.173/toolchain/.

Switch the container image so that things work correctly.

Task: T7573