gh-actions: don't try to use ppa

Author: nschloe

.github/workflows/ci.yml

@@ -37,9 +37,9 @@ jobs:
     - name: Install CGAL 5
       run: |
         # 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-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: |