[feature][ci] also check clang.

.gitlab-ci.yml

@@ -34,9 +34,9 @@ debian-11-gcc-9-17:
     DUNECI_TOOLCHAIN:  gcc-9-17
     DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=OFF -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"
 
-ubuntu-20.04-gcc-9-17:
+ubuntu-20.04-clang-10-17:
   image: registry.dune-project.org/docker/ci/ubuntu:20.04
   script:
     - duneci-standard-test
   variables:
-    DUNECI_TOOLCHAIN:  gcc-9-17
+    DUNECI_TOOLCHAIN:  clang-10-17