Report zipkin spans regardless of goal version. #7915
Goals which use the v2 engine were not exporting zipkin spans because the code doing the exporting was inside the context of the legacy graph, which isn't used during v2 engine goal execution.
Moved the zipkin bulk export code to the local pants runner so that it gets called regardless of which engine is run.
Users can now see spans / timings from engine workunits displayed through zipkin dash for both v1 and v2 goals.