Don't build notebooks on ubuntu 20.04; 22.04 only

.github/workflows/ubuntu-system-ci.yml

@@ -115,7 +115,8 @@ jobs:
         fi
 
     # Don't do this in the `Build BioDynaMo step`, because notebooks might time out in GHA
-    # runners, while waiting for other targets to be compiled
+    # runners, while waiting for other targets to be compiled. Run notebooks only
+    # if the event is a pull request, and on ubuntu-22.04.
     - name: Run notebooks
       shell: bash
       run: |
@@ -131,7 +132,8 @@ jobs:
           -Dreal_t=${{ env.REALT }} \
           -B build
         cmake --build build --parallel --config Release --target notebooks  -- -j1
-      if: github.event_name == 'pull_request'
+      if: github.event_name == 'pull_request' && matrix.os == 'ubuntu-22.04'
+
 
     - name: Build BioDynaMo
       shell: bash