Update build-windows.yml

.github/workflows/build-windows.yml

@@ -29,6 +29,11 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v4
 
+    - name: Setup
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
     - name: Build
       run: |
         cd v2rayN