github: s/clang++-18/clang++/

in 28b0555, we switched from ubuntu:jammy to fedora:40 as the
building environment of our CI workflow. and we replaced the
input parameter "clang++-18" in test.yml with "clang++", because
in fedora:40, the clang++ executable does not have the postfix
as the clang-18 installed by the setup-cpp action.

but we failed to update all of them in that change, that's why
the "check header" step was not performed after that change.

in this change, we replace that last "clang++-18" in the
workflow to "clang++".

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

.github/workflows/test.yaml

@@ -86,7 +86,7 @@ jobs:
         run: cmake --build build/${{inputs.mode}}
 
       - name: Check Header
-        if: ${{ inputs.mode == 'dev' && inputs.compiler == 'clang++-18' }}
+        if: ${{ inputs.mode == 'dev' && inputs.compiler == 'clang++' }}
         run: cmake --build build/${{ inputs.mode }} --target checkheaders
 
       - name: Build with C++20 modules