Fix wrong variable name in BuildProcessorInterface::build()

Hi khiminrm,

I confirmed the issue was resolved when I applied patch #2 to the "Facets" module against Version 2.0.x-dev.

Please see the screenshots attached.

For your review.
Thank you.

But that's not the case throughout. There are other plugins using BuildProcessorInterface

Do we consider this to be a BC break and as such can it only go into 3.x?