modules/config/fonts.nix: export GDFONTPATH in /etc/profile

Set the environment variable GDFONTPATH to the list of directories where fonts
are expected to be installed. This is search path is used by same GD-based
applications, such as Gnuplot.

Alternatively, we could rely on fontconfig to achieve the same thing, i.e. by
running

  for n in $(fc-list | sed -r -e 's|^([^:]+):.*$|\1|'); do echo $(dirname "$n"); done | sort | uniq

to get that list at run-time, but the static approach feels more deterministic.