try using MSVC toolchain

Author: mamoreau-devolutions

.github/workflows/halide-prebuilt.yml

@@ -109,7 +109,7 @@ jobs:
           }
 
           echo "LLVM_HOST_BIN_DIR=$LLVM_BIN_DIR" >> $Env:GITHUB_ENV
-          echo "PATH=$LLVM_BIN_DIR$([IO.Path]::PathSeparator)$Env:PATH" >> $Env:GITHUB_ENV
+          #echo "PATH=$LLVM_BIN_DIR$([IO.Path]::PathSeparator)$Env:PATH" >> $Env:GITHUB_ENV
 
       - name: Install target clang+llvm
         if: matrix.arch == 'aarch64'
@@ -204,6 +204,7 @@ jobs:
         shell: pwsh
         run: |
           if ($IsWindows) {
+            Enter-VsDevShell x64
             $TargetArch = @{'x86_64'='x86_64';'aarch64'='arm64'}['${{matrix.arch}}']
             $Env:CC="clang-cl --target=${TargetArch}-pc-windows-msvc"
             $Env:CXX="clang-cl --target=${TargetArch}-pc-windows-msvc"