Fix stdlib=libc++ condition for clang++-19 on Ubuntu

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Wentzell

.github/workflows/build.yml

@@ -112,7 +112,7 @@ jobs:
         echo "LDFLAGS=-L$(brew --prefix llvm)/lib/c++ -L/opt/homebrew/opt/llvm/lib/unwind -lunwind" >> $GITHUB_ENV
 
     - name: Add clang CXXFLAGS
-      if: ${{ matrix.cxx == 'clang++' }}
+      if: ${{ startsWith(matrix.cxx, 'clang') }}
       run: |
         echo "CXXFLAGS=-stdlib=libc++" >> $GITHUB_ENV