This is mostly a note to self, since I always end up forgetting about this and I have proven myself unable to actually find this information easily for whatever stupid reason.
key-build and friends are generated by the base-files package build. To recreate them:
rm key-build* OR MOVE THEM SOMEWHERE ELSE FOR STORAGE
make package/base-files/clean
make -j
I have not checked whether this is enough for the image-builder and other incarnations of build keys.