Update nuget.yaml (#522)

Author: tg123

.github/workflows/nuget.yaml

@@ -21,6 +21,10 @@ jobs:
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: '3.1.x'
+    - name: Setup dotnet SDK 5
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '5.0.x'          
     - name: Test
       run: dotnet test
     - name: Pack