fix: github CI

Author: lukasrothenberger

.github/workflows/ci.yml

@@ -111,7 +111,7 @@ jobs:
       - name: C++ unit tests
         run: |
           build/test/unit_tests/DiscoPoP_UT
-  
+
   execute_end_to_end_tests:
     runs-on: ubuntu-20.04
     name: Execute end-to-end tests
@@ -158,6 +158,7 @@ jobs:
 
       - name: Python end-to-end-tests
         run: |
+          . venv/bin/activate
           python -m unittest -v -k *.end_to_end.*
   update_wiki-build:
     name: "Update Wiki - Build"