CI: install graphviz

Author: fingolfin

.github/workflows/CI.yml

@@ -38,7 +38,7 @@ jobs:
       - name: 'Install additional dependencies'
         run: |
           sudo apt-get update
-          sudo apt-get install polymake singular
+          sudo apt-get install polymake singular graphviz
       - uses: gap-actions/setup-gap@v2
         with:
           GAP_PKGS_TO_CLONE: "polymaking hapcryst" # We need >= 0.8.4 and 0.1.13 to support polymake 4.0