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 Provide an option to check whether the modelfile needs to be regenerated each time, as the modelfile may become outdated.

[gmarav05]

Hi @chlins ,

I recently submitted a small CLI usability PR and was looking at this issue next.

My understanding:

1. modctl build should automatically generate a Modelfile if one is not present
2. Optionally allow users to force regeneration when the Modelfile is outdated

Can you please assign me this issue?

[chlins]

@gmarav05 Thanks for picking this up! I've assigned to you.