This is an interesting idea, but unfortunately it’s not something Buildkite supports at this stage.
This feature request, it may be worth combining your votes on there to help raise it’s profile.
For now, @xiaket has suggested a good option that we support. The agents can have environment variables set in the environment hook(https://buildkite.com/docs/agent/v3/hooks), and depending on the variety of agents you’re running it may be feasible to set environment variables in a small number of hook scripts to add them into all builds.