Update Windows build to include CMAKE_PREFIX_PATH for LLVM

Author: allnes

.github/workflows/main.yml

@@ -865,6 +865,7 @@ jobs:
           -D CMAKE_CXX_COMPILER_LAUNCHER=ccache
           -D CMAKE_BUILD_TYPE=Release
           -D CMAKE_INSTALL_PREFIX=install
+          -D CMAKE_PREFIX_PATH="C:/Program Files/LLVM"
         env:
           CC: clang-cl
           CXX: clang-cl