Add caching of pictures in CI

Author: WoWaster

.github/workflows/main.yml

@@ -20,6 +20,12 @@ jobs:
       - name: Set up Git repository
         uses: actions/checkout@v4
 
+      - name: Cache TikZ externalized pictures
+        uses: actions/cache@v4
+        with:
+          path: tex/figures/externalized
+          name: tikz-externalized
+
       - name: Compile LaTeX document
         uses: xu-cheng/latex-action@v3
         with: