ci: clarify instrumentation failure impact
Created by: jhchabran
Twice today, someone got confused and incorrectly assumed the failure in logs uploading was the cause of a failed build.
The problem is that leads teammates to think the CI is at fault, while it actually isn't, which will either be surfaced by a subsequent failure or reaching us on the support.
This PR adds a visible warning explaining that the log uploading and trace uploading are not the source of build failures.
Test plan
See QA run: https://buildkite.com/sourcegraph/sourcegraph/builds/141537#fd1313e3-29e1-4d61-a9b5-6eac8e495ae5/174-176 (I have adjusted the message since then, the unnecessary :
are not present anymore).