The ubuntu-18.04 runner needs to be updated in the test actions

[ajkavanagh]

Essentially, the tests don't run due to ubuntu-18.04 being requested which has been deprecated: https://github.com/actions/runner-images/issues/6002 and was removed on the 2023/4/1.

Description seen against a test:

Requested labels: ubuntu-18.04 Job defined at: juju/charm-helpers/.github/workflows/build.yml@refs/pull/769/merge Waiting for a runner to pick up this job...

It will then wait indefiniately.