GHA: download the SDK to the proper location

We would previously download the AMD64 SDK to the wrong location and
would thus fail to find the necessary content.

Cherry pick commit e108a13

Author: hjyamauchi

.github/workflows/swift-toolchain.yml

@@ -2735,7 +2735,7 @@ jobs:
       - uses: actions/download-artifact@v4
         with:
           name: Windows-sdk-amd64
-          path: ${{ github.workspace }}/BinaryCache
+          path: ${{ github.workspace }}/BuildRoot/Library/Developer/Platforms/Windows.platform
       - name: Download SDK
         uses: actions/download-artifact@v4
         with: