ci: annotate when an error also occurred on main

[nrainer-materialize]

Demo (expecting a failure in "unused dependencies"): https://buildkite.com/materialize/nightlies/builds?branch=nrainer-materialize%3Aci%2Fannotate-error-also-on-main

[nrainer-materialize]

Some observations:

• As of now, this will only work for mzcomposed jobs because it is called from ci/plugins/mzcompose/hooks/pre-exit (that is, it won't work for jobs like "Unused dependencies")
• It requires a Buildkite API Access Token.

[nrainer-materialize]

If this creates too many requests, I will need to check if I can get the list of all failing build steps in "Analyze tests" at the end (instead of sending a request for each failing build step).