I tend to disagree with this as it seems like an ad for Nix/Buildkite...

If your CI invocations are anything more than running a script or a target on a build tool (make, etc.) where the real build/test steps exist and can be run locally on a dev workstation, you're making the CI system much more complex than it needs to be.

CI jobs should at most provide an environment and configuration (credentials, endpoints, etc.), as a dev would do locally.

This also makes your code CI agnostic - going between systems is fairly trivial as they contain minimal logic, just command invocations.