don't use cgal from ppa

Author: nschloe

.github/workflows/ci.yml

@@ -34,11 +34,12 @@ jobs:
     - uses: actions/checkout@v2
       with:
         lfs: true
-    - name: Install CGAL 5 from PPA
+    - name: Install CGAL 5
       run: |
-        sudo apt-get install -y software-properties-common
-        sudo apt-add-repository -y ppa:nschloe/cgal-backports
-        sudo apt update
+        # Leave that here in case we ever need a PPA again
+        # sudo apt-get install -y software-properties-common
+        # sudo apt-add-repository -y ppa:nschloe/cgal-backports
+        # sudo apt update
         sudo apt install -y libcgal-dev
     - name: Install other dependencies
       run: |