Spawn challenges with a random flag when possible #19
Description
Now that we support copy_flag_using_build_arg (#18), we could re-build the challenge every time with a random flag.
Note:
- Right now we'd re-clone the challenge (so we should probably do it only if we have a deploy key or the full repo is public)
- For some challenges it can take a long time to rebuild.
- Right now we create a new checkout at every rebuild. If we do it continuously the private view (and the DB) would get kinda dirty with many auto-created checkouts. Perhaps we should refactor to have "Build" objects associated with each ChalCheckout.