Move Ubuntu 18.04 builds into earlier build stage
GitLab.com will be updating the hosts to Ubuntu 18.04 (https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6539), so we should make sure these packages are available with Ubuntu 16.04.
Sample pipeline on dev: https://dev.gitlab.org/gitlab/omnibus-gitlab/pipelines/110489
Edited by GitLab Release Tools Bot