ci, fix coverage job Clang_DIR: clang-18 suffix breaks find_package(Clang)

[steven-varga]

Problem

The coverage job in .github/workflows/ci.yml computes clang_cmakedir with a version suffix:

clang_cmakedir="$(dirname "${llvm_cmakedir}")/clang-18"

The directory /usr/lib/llvm-18/lib/cmake/clang-18 does not exist on Ubuntu; the correct path is /usr/lib/llvm-18/lib/cmake/clang (no version suffix). This causes find_package(Clang CONFIG) to silently fall back to a system stub and the build fails.

The build matrix jobs already carry the correct form (fixed in PR #17):

clang_cmakedir="$(dirname "${llvm_cmakedir}")/clang"

Fix

Remove the -18 suffix on line 433 of the coverage Configure step to match the build matrix.