Reflect change to clang 7 also in job name not only in image choice

.gitlab-ci.yml

@@ -18,7 +18,7 @@ git-debian-10-gcc-7-14:
     - "cores:20"
     - "perftool"
 
-git-debian-10-clang-6-libcpp-17:
+git-debian-10-clang-7-libcpp-17:
   image: "registry.dune-project.org/docker/ci/dune-pdelab:git-debian-10-clang-7-libcpp-17"
   script: duneci-standard-test
   tags: