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.

Author: jdtournier

.github/workflows/checks.yml

@@ -84,7 +84,6 @@ jobs:
     - name: install dependencies
       run: |
          brew update || brew update     # https://github.com/Homebrew/brew/issues/2491#issuecomment-294207661
-         brew upgrade
          brew install qt5 eigen pkg-config libtiff fftw libpng
          brew link --force qt5