build.yml: ensure 'latexmk' is installed

Author: sommersoft

.github/workflows/build.yml

@@ -36,7 +36,7 @@ jobs:
     - name: Install deps
       run: |
         sudo apt-get install -y eatmydata
-        sudo eatmydata apt-get install -y gettext librsvg2-bin mingw-w64
+        sudo eatmydata apt-get install -y gettext librsvg2-bin mingw-w64 latexmk
         pip install requests sh click setuptools cpp-coveralls "Sphinx<4" sphinx-rtd-theme recommonmark sphinx-autoapi sphinxcontrib-svg2pdfconverter polib pyyaml astroid isort black awscli
     - name: Versions
       run: |