[proposal] --output-snapshot-on-exit should emit a snapshot on timeout

[DerekTBrown]

Describe the Feature You Want

• If --output-snapshot-on-exit is set, tilt ci should output a snapshot when a timeout is reached.

Current Behavior

• tilt ci does not emit a snapshot.

Why Do You Want This?

• We use tilt snapshot to help developers debug issues on CI systems, since they cannot access the Tilt UI.
• Snapshots are useful to being able to debug why a tilt ci run is timing out, for instance, a dependency ordering issue or a service not successfully starting up.