chore(cli): support cobra completion command (#2217)
We currently support https://github.com/spf13/cobra/blob/main/shell_completions.md by default because of using Cobra, but the default run command currently breaks this. This PR allows this feature to continue functioning as intended.
_**As an alternative,**_ we could easily disable this because it's likely not super commonly used. This would also allow `completion` pipelines to work without needing `run`.
Fixes https://github.com/vercel/turborepo/issues/2207
