Update old host images

[ViktorHofer]

Ubuntu jobs use Ubuntu20.04 and the source-build job uses mariner-2. Ideally command-line-api would just use the defaults that are kept up-to-date.

• command-line-api/.vsts-ci.yml

  Line 83 in 35ab6f0

  demands: ImageOverride -equals 1es-ubuntu-2004-open

[ViktorHofer]

command-line-api should also import pool-providers.yml from arcade to not hardcode the pool names.