Fix macOS CI to use Homebrew LLVM clang

Use the full path to Homebrew LLVM's clang instead of relying on
the system clang. AppleClang doesn't have the Clang development
headers (clang/Tooling/Tooling.h) needed to build rellic.

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

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

Author: artemdinaburg

.github/workflows/build.yml

@@ -91,9 +91,10 @@ jobs:
       - name: Install LLVM
         run: |
           brew install llvm@${{ matrix.llvm }} ninja
-          echo "CC=clang" >> $GITHUB_ENV
-          echo "CXX=clang++" >> $GITHUB_ENV
-          echo "LLVM_DIR=$(brew --prefix llvm@${{ matrix.llvm }})/lib/cmake/llvm" >> $GITHUB_ENV
+          LLVM_PREFIX=$(brew --prefix llvm@${{ matrix.llvm }})
+          echo "CC=$LLVM_PREFIX/bin/clang" >> $GITHUB_ENV
+          echo "CXX=$LLVM_PREFIX/bin/clang++" >> $GITHUB_ENV
+          echo "LLVM_DIR=$LLVM_PREFIX/lib/cmake/llvm" >> $GITHUB_ENV
 
       - name: Checkout
         uses: actions/checkout@v4