HomeBrew: don't update the full install
HomeBrew: don't update the full install

This step takes a long time, is prone to failure, and unnecessary at the
moment - we can rely on the default install being sufficiently up to
date.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3