Explicitly use 'stable' for default value of channel args
Explicitly use 'stable' for default value of channel args

This will resolve an issue where the new `--auto-update` flag was
incorrectly automatically retrieving updates from the unstable
channel by default. This change should additionally prevent mistakes
like this from escaping in the future by explicitly requiring a channel
and more clearly stating how our default channel is calculated.

Signed-off-by: Jamie Winsor <b4dde37aa2aac745c0e6c43447507eaeace3e59f@vialstudios.com>

rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat