Consider adding a CLI command for refetching NStack

[nivisi]

We can use a separate CLI command, like nstack refetch instead of builders. This way we'll save time:

• We won't rerun the whole code generation just to update a string. All we'll need to do is nstack refetch
• We won't include NStack in regular code generation — again, saving some time.

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To Do:

• Agree on the CLI command name
• Figure out how it can be implemented
• Implement it

[nivisi]

cc @johsoe