nuget yml file update

Author: swagfin

.github/workflows/release-nuget-pkg.yml

@@ -13,6 +13,8 @@ jobs:
       uses: actions/checkout@v4
     - name: Setup .NET SDK
       uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '8.0.x'      
     - name: Build
       run: dotnet build ObjectSemantics.NET/ObjectSemantics.NET.csproj -c Release
     - name: Test