Skip build-nuget CI job when PR originates from a local branch

.github/workflows/ci.yml

@@ -35,6 +35,7 @@ jobs:
 
  # Build the nuget package and upload it as an artifact.
  build-nuget:
+ if: github.event_name == 'push' || github.event.pull_request.head.repo.id != github.event.pull_request.base.repo.id
  name: Build NuGet package
  runs-on: ubuntu-latest
  steps: