Install pre-requisites for magick package (and hence gdiff package)

[hturner]

When making changes to graphical functions, contributors are often asked to do graphical difference testing using the gdiff package (as in this example: r-devel/r-project-sprint-2023#74 (comment)).

gdiff depends on the magick package, which has ImageMagick as a system dependency.

We should update the container to install this - it can be installed via libmagick++-dev on ubuntu

[hturner]

fixed by #251