Do not install graphviz on the LaTeX container image

graphviz is pre-installed, so there's no point installing it.

.github/workflows/main.yml

@@ -129,8 +129,6 @@ jobs:
         python-version: "3"
     - name: Check Python version
       run: python --version
-    - name: Install graphviz
-      run: sudo apt-get install graphviz
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip