[!583] [ci] Switch to clang 7 for libc++ build

Merge branch 'ci/clang-7' into 'master'

See merge request [!583]

  [!583]: Nonecore/dune-common/merge_requests/583

.gitlab-ci.yml

@@ -27,11 +27,11 @@ debian:10 gcc-8-noassert-17:
     DUNECI_CMAKE_FLAGS: "-DDUNE_PYTHON_VIRTUALENV_SETUP=1"
   tags: [duneci]
 
-debian:10 clang-6-libcpp-17:
+debian:10 clang-7-libcpp-17:
   image: registry.dune-project.org/docker/ci/debian:10
   script: duneci-standard-test
   variables:
-    DUNECI_TOOLCHAIN: clang-6-libcpp-17
+    DUNECI_TOOLCHAIN: clang-7-libcpp-17
   tags: [duneci]
 
 debian:9 gcc-6-14: