Fix pipeline author for Slack and use pipeline id for pipeline link

[Context](https://gitlab.slack.com/archives/runner-builds/p1479238493000018)

Pipeline Slack message:

>  gitlab-org/gitlab-ci-multi-runner: Pipeline 8bed2a3b of fix/handle-failed-state-while-patching-trace branch by  failed in 1389 seconds 

Quotes:

> @godfat Take a look at these notifications, they seems to be missing author and should probably use Pipeline ID instead of SHA

See merge request !7506