actually define DUNE_CMAKE_FLAGS and not only DUNECI_CMAKE_FLAGS

.gitlab-ci.yml

 ---
 variables:
   # dune-python will use DUNE_OPTS when building stuff
-  DUNECI_CMAKE_FLAGS: "-DDUNE_ENABLE_PYTHONBINDINGS=ON -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
-  DUNECI_TOOLCHAIN:  gcc-7-17
-  DUNE_CONTROL_PATH: /duneci/modules:$CI_PROJECT_DIR
-  DUNE_PY_DIR:       /duneci/modules/dune-py
-  PYTHONPATH:        "/duneci/modules/dune-common/build-cmake/python:/duneci/modules/dune-geometry/build-cmake/python:/duneci/modules/dune-grid/build-cmake/python:/duneci/modules/dune-istl/build-cmake/python:/duneci/modules/dune-alugrid/build-cmake/python:/duneci/modules/dune-fem/build-cmake/python:$CI_PROJECT_DIR/build-cmake/python"
+  DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=ON -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
+  DUNECI_CMAKE_FLAGS: $DUNE_CMAKE_FLAGS
+  DUNECI_TOOLCHAIN:   gcc-7-17
+  DUNE_CONTROL_PATH:  /duneci/modules:$CI_PROJECT_DIR
+  DUNE_PY_DIR:        /duneci/modules/dune-py
+  PYTHONPATH:         "/duneci/modules/dune-common/build-cmake/python:/duneci/modules/dune-geometry/build-cmake/python:/duneci/modules/dune-grid/build-cmake/python:/duneci/modules/dune-istl/build-cmake/python:/duneci/modules/dune-alugrid/build-cmake/python:/duneci/modules/dune-fem/build-cmake/python:$CI_PROJECT_DIR/build-cmake/python"
 
 before_script:
   - python3 -m venv /duneci/modules/dune-pip