CI: ubuntu-18.04 brownouts

While running the docker-image task in https://github.com/oxidecomputer/omicron/actions/runs/2905040077:

> `This is a scheduled Ubuntu-18.04 brownout. The Ubuntu-18.04 environment is deprecated and will be removed on December 1st, 2022. For more details, see https://github.com/actions/runner-images/issues/6002`