v2020: Sync first stage config with v2021dev3

All the release channels now use the same first stage based on the v2020 NFS artifacts rather than the old ones based on 18.12.

This means that tests are currently broken because LAVA fails to detect the login prompt as the string changed.

Update the config using what is being used by v2021dev3.

See also https://gitlab.apertis.org/pkg/development/apertis-tests/-/merge_requests/204 for v2019.

Test run here: https://lava.collabora.co.uk/scheduler/job/2565713