fix: Add missing libclang-rt-18-dev for coverage instrumentation

The libclang-rt-18-dev package is required for LLVM 18 coverage
instrumentation to work properly. It provides the compiler runtime
libraries (libclang_rt.profile-x86_64.a) needed during linking when
using -fprofile-instr-generate and -fcoverage-mapping flags.

Without this package, linking fails with:
  cannot find /usr/lib/llvm-18/lib/clang/18/lib/linux/libclang_rt.profile-x86_64.a

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Haiku 4.5 <[email protected]>

Author: cdecker

.github/scripts/setup.sh

@@ -18,6 +18,7 @@ sudo apt-get -qq install --no-install-recommends --allow-unauthenticated -yy \
      binfmt-support \
      build-essential \
      clang-18 \
+     libclang-rt-18-dev \
      cppcheck \
      docbook-xml \
      eatmydata \