custom_build not handling cancellation correctly

[nicks]

Steps to Reproduce

Use the following tiltfile:

custom_build('example-html-image', 'sleep 1000', deps=[])
k8s_yaml('kubernetes.yaml')
k8s_resource('example-html', port_forwards=8000)

(adapted from tilt-example-html)

Run tilt up

Hit the cancel button on example-html

Expected Behavior

The sleep should get killed

Current Behavior

The sleep is not killed properly.