Introduce caching of files

Caching can be an important performance boost.
With no caching, LaTeX has to regenerate all auxiliary files,
including glossary and bib files, every time.
This also applies to SVG files that were automatically
converted to PDF using the svg package (with inkscape
in the background). Discarding these PDFs each time is also
costly.

Existing auxiliary files help latexmk finish in fewer runs.

In a first attempt, caching reduced pipeline run times by 50%.

.gitlab-ci.yml

@@ -39,6 +39,9 @@ build_latex:
  stage: build
  script:
  - make tex
+ cache:
+ untracked: true
+ key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
 
 build_pandoc:
  stage: build