CI: Disable container image builds on Ubuntu

The kayobe-seed-ubuntu-focal job is currently fairly unreliable, often
failing to build the base container image.

We are not using the mirrors provided by OpenDev infra, which may be
making these builds less reliable.

This change disables container image builds in CI on Ubuntu. It should
be reverted if they are made more reliable.

Change-Id: I648fa6423ad9ff43120c7808f080b0359ad8621c