BinaryBuilder.jl: how to select gcc 9.1.0

I have a binary executable that needs to be compiled with gcc 9.1.0. I see in the code of BinaryBuilderBase.jl that there is provision for gcc 9.1.0, but I don’t undertstand how I can request such a shard. Selecting libgfortran_version=v"5.0.0" and libstdcxx_version = v"3.4.26" doesn’t seem to be sufficient: I still get only gcc -v 8.1.0

build_tarballs(..., preferred_gcc_version=v"9")

Thanks a lot.