use clang for clang-tidy job

And add warnings for gcc AND clang builds (as they can differ slightly).

Clang does not support -Wused-but-set-parameter yet, though
support is [on the way](https://reviews.llvm.org/D100581).
So this gets rid of this warning we see in CI:

> error: unknown warning option '-Wno-unused-but-set-parameter'; did you mean '-Wno-unused-parameter'?

Author: v4hn

.github/workflows/ci.yaml

@@ -18,13 +18,15 @@ jobs:
             NAME: ccov
             TARGET_CMAKE_ARGS: -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_CXX_FLAGS="--coverage"
           - IMAGE: master-source
-            NAME: clang
-            CXX: clang++
+            CXXFLAGS: >-
+             -Werror -Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls
+             -Wno-unused-parameter -Wno-unused-function -Wno-deprecated-copy -Wno-unused-but-set-parameter
           - IMAGE: noetic-source
+            CXX: clang++
             CLANG_TIDY: true
             CXXFLAGS: >-
              -Werror -Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls
-             -Wno-unused-parameter -Wno-unused-function -Wno-deprecated-copy -Wno-unused-but-set-parameter
+             -Wno-unused-parameter -Wno-unused-function -Wno-deprecated-copy
           - IMAGE: noetic-source
             NAME: asan
             DOCKER_RUN_OPTS: "-e PRELOAD=libasan.so.5"