Update .gitlab-ci.yml

.gitlab-ci.yml

@@ -77,6 +77,9 @@ stages:
 gcc_9_mpionly:
    extends: .build_template
    stage: build
+   before_script:
+      - export CXX=g++
+      - export CC=gcc
    variables:
       CTEST_EXCLUDE_LABELS: "longrun"
       WALBERLA_BUILD_WITH_MPI: "ON"