[cache-flag] Warn when attempting to configure launch cache image

[jromero]

Summary

Launch cache is really only useful for local builds in the format of bind or volume otherwise the final app image already contains the launch layers. Due to that fact, trying to create an additional launch cache image doesn't make sense (or at least we don't have a good use case for it). In this scenario, we would still want to notify the user, in the event that they were trying to achieve something that is unsupported or if they misconfigured the cache flag.

Acceptance Criteria

Image launch cache

Given the current working directory contains source code
And configured builder can build source code
When user executes 'pack build myorg/myapp:latest --cache "type=launch;format=image;name=myorg/myimage:cache"'
Then the build should succeed
And a warning containing the following should be present:
"""
ignoring cache definition: 'launch' cache in format 'image' is not supported.
"""