Licenses for CI build machines

Does anyone know if it is possible to buy special CI seats when using Unity Pro?

Something that could not run the editor but could be used in CI, using the headless command line interface.

I've seen some posts that suggest that this might be possible but nothing in the Unity Store.

We're currently using our spare normal seats in our CI but would prefer to buy a special (slightly cheaper) licence that would allow us to run Unity build scripts in CI. We're not able to use Unity Cloud build.