Add --skip-duplicate flag to NuGet push (#421)

This prevents the pipeline from failing when attempting to republish an already published package version.

Author: Aaronontheweb

.github/workflows/publish_nuget.yml

@@ -30,7 +30,7 @@ jobs:
       run:  dotnet pack -c Release -o ./output
 
     - name: Push Packages
-      run: dotnet nuget push "output/*.nupkg" -k ${{ secrets.NUGET_KEY }} -s https://api.nuget.org/v3/index.json
+      run: dotnet nuget push "output/*.nupkg" -k ${{ secrets.NUGET_KEY }} -s https://api.nuget.org/v3/index.json --skip-duplicate
 
     - name: release
       uses: actions/create-release@v1