Post a note on the upstream project's commit that triggered the pipeline

This is the equivalent of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21512 but for omnibus-gitlab.

Second part of gitlab-org/gitlab-ce#47818.

This uses a new TOP_UPSTREAM_TRIGGER_PROJECT env variable passed from the CE package-and-qa job, this is done by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22110. That said, this MR can be merged before https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22110 since it won't try to post a comment if TOP_UPSTREAM_TRIGGER_PROJECT isn't present.

This also forward this variable to the triggered gitlab-qa pipeline so that gitlab-qa will be able to finally post a comment on the CE/EE commit under test (3rd step of gitlab-org/gitlab-ce#47818).

Closes gitlab-org/gitlab-ce#47818.