https://buildkite.com/docs/agent/v3/hooks indicates you can override the builtin git support with your own checkout system. Sucks having to write a plug-in but it should be short.

Somebody already has, for perforce:

https://github.com/improbable-eng/perforce-buildkite-plugin

I've looked into this plugin before (we also use perforce). This does techincally let you build from perforce, in the same way making `p4 sync` your first build step does.