Call sky down when cancelling skypilot experiment task

[hemildesai]

From #340 (comment)

There's open questions about how to handle ctrl+c and cancelling sky launch requests through nemo run. Current behavior appears to be nemo experiment cancel just cancels the task, but does not sky down the cluster. When coupled with retry_until_up=True, the sky launch request may keep running indefinitely and will need to be manually terminated with sky down or a sky api cancel.
We may want to have sky down run when nemo experiment cancel is run