Update dotnet-ci.yaml

Author: JusterZhu

.github/workflows/dotnet-ci.yaml

@@ -11,7 +11,7 @@ jobs:
     - name: Setup .NET SDK
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '8.0.x'  
+        dotnet-version: '8.0.100-rc.2.23502.2'
     - name: Restore dependencies
       run: dotnet restore ./src/WPF/GeneralUpdate.Tool.sln
     - name: build