Simplify the user experience for build

[chlins]

Currently, when users want to build a model artifact, they must first execute modctl modelfile generate followed by modctl build. We should simplify this workflow. Here is an initial idea for optimization:

1. Users will only need to execute modctl build. If no modelfile is found, it should be generated automatically.
2. Provide an option to check whether the modelfile needs to be regenerated each time, as the modelfile may become outdated.