Look for passing commits in the Security repository
Commits#latest_successful_on_build will always fetch commits from the
Security repository, whereas before it would use the Build repository
during a security release.
Similarly, during a security release PassingBuild will no longer
naively grab the latest commit in the ref branch, but instead will
require a full, successful pipeline just as it does for a normal
release, because the Security repository will run pipelines just like
Canonical.
This aims to resolve gitlab-com/gl-infra/delivery#625 (closed).
Edited by Robert Speicher