Ahh, sorry, I thought you want to download it during the build.
For the API: the new API does not have that endpoint yet, but definitely will soon. For this please vote & comment here: Backup workflow via Bitrise API we’ll announce it there when this is available.
There’s still an older API endpoint where you can get the yml, but that’s pretty much deprecated by now and we’ll remove that once the new API endpoint is ready. If you’d need that before the new endpoint is ready, you can find the related guide & docs at: http://devcenter.bitrise.io/troubleshooting/workflow-editor-doesnt-load/