Update build.yml

Author: koenbeuk

.github/workflows/build.yml

@@ -24,7 +24,7 @@ jobs:
     - name: Test
       run: dotnet test --no-build --verbosity normal
     - name: Pack
-      run: dotnet pack -v normal -c Release --include-symbols --include-source
+      run: dotnet pack -v normal -c Release --include-symbols --include-source -o nupkg
     - name: Upload Artifact
       uses: actions/upload-artifact@v2
       with: