Because cscore has been delayed more than I had anticipated, I've uploaded a build of mjpg-streamer. See
https://github.com/robotpy/roborio-packages
mjpg-streamer is clunky to manage, and once cscore is available, you'll find that migrating to it from mjpg-streamer should be fairly straightforward and will offer a number of advantages (including integration with the default dashboards). But, this will get you going for now.