CI: fix msan libc++ build — LIBCXXABI_USE_LLVM_UNWINDER=OFF

The msan lane rebuilds an MSan-instrumented libc++ from LLVM 18.1.8 source
when the cache misses. LLVM 18 defaults LIBCXXABI_USE_LLVM_UNWINDER=ON, which
now hard-errors at libcxxabi/CMakeLists.txt:51 unless libunwind is also in
LLVM_ENABLE_RUNTIMES:

  LIBCXXABI_USE_LLVM_UNWINDER is set to ON, but libunwind is not specified
  in LLVM_ENABLE_RUNTIMES.

The prior green runs hit the libc++ cache and never re-ran this config, so the
latent breakage only surfaced on cache eviction. Set the flag OFF (the
instrumented libc++ uses the system unwinder, sufficient for running the test
suite) rather than pulling libunwind into the instrumented runtimes build.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NpysYDDJac63yz2mZKKiDf

Author: etr

.github/workflows/verify-build.yml

@@ -699,6 +699,7 @@ jobs:
           -DCMAKE_C_COMPILER=clang-18 \
           -DCMAKE_CXX_COMPILER=clang++-18 \
           -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
+          -DLIBCXXABI_USE_LLVM_UNWINDER=OFF \
           -DLLVM_USE_SANITIZER=MemoryWithOrigins \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -DCMAKE_INSTALL_PREFIX="${{ github.workspace }}/libcxx-msan" ;