Missing build / test targets doesn't fail build

[keith]

https://buildkite.com/bazel/rules-cc/builds/4592/steps/canvas?sid=019be24d-c9c9-4dbc-a1a2-b76ccb1df8b9

accidentally just had this:

  ubuntu_bzlmod:
    name: Ubuntu 20.04 (Bazel LTS, bzlmod)
    bazel: last_rc # TODO: change to 9.x when released
    platform: ubuntu2004
    build_flags:
      - "--enable_bzlmod"
      - "--ignore_dev_dependency"