pack only when releasing

Author: ssg

.github/workflows/build.yml

@@ -31,6 +31,7 @@ jobs:
       run: dotnet test --no-build --configuration Release
 
     - name: Pack
+      if: startsWith(github.ref, 'refs/tags/')
       run: dotnet pack src --no-build --configuration Release --output ./nupkg
 
     - name: Publish to NuGet