ci: Explicitly specify x64 MSBuild

CI builds are currently failing due to x86 host tools getting selected
for x64 targets.

.github/workflows/build.yml

@@ -31,6 +31,8 @@ jobs:
 
       - name: Add msbuild to PATH
         uses: microsoft/setup-msbuild@v2
+        with:
+          msbuild-architecture: x64
 
       - name: Build
         run: |