There's no need for us to recompile deutex every time we build Freedoom,
since it's already in Github Actions. In a comment on #671 it was
mentioned that this was done to work around a crash in the version of
deutex in bionic, but that bug is long-fixed now.
This is not invoked as part of the normal build process, and shouldn't
be: we only ever want to build the cover PDFs when we're uploading them
for printing, and don't want everyone who wants to build Freedoom to
have to install Inkscape. However, we should nonetheless include it in
the CI to check that the build step works.
