OK, I see... 

Problem is that this is part of the 'API', although a very specific part (i.e. only easyblocks that derive from `CMakeMake` are affected).

I guess it's OK to fix, as long as you also fix the Geant4 easyblock that uses the `builddir` argument named. 
We'll properly document it in the release notes (so maybe already add a preliminary note there so it's not forgotten).