ligGL now included in docker image. Use cache from gcc_9 for Python

.gitlab-ci.yml

-image: irmb/virtualfluids-python-deps
+image: irmb/virtualfluids-python-deps:latest
 
 stages:
   - build
@@ -166,7 +166,7 @@ gcc_9_python:
   needs: ["gcc_9"]
 
   cache:
-    key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
+    key: "gcc_9-$CI_COMMIT_REF_SLUG"
     paths:
       - $BUILD_FOLDER
 
@@ -217,7 +217,6 @@ gcc_9_python_bindings_test:
   before_script:
     - export PYTHONPATH="Python"
     - export VF_WHEEL=$(find dist/*.whl)
-    - apt-get update && apt-get install -y libgl1
     - pip3 install $VF_WHEEL
     - pip3 install -r Python/requirements.txt