update cobra pkg and make use of new completion features

[fho]

https://github.com/spf13/cobra/releases

• support generating and installing bashcompletions for zsh and fish shell
• disable the default "completions" subcommand registered in the new cobra release

CLI could be: baur init completion --zsh|--bash|--fish [--global|--local|--stdout]

• --local installs the the completion script to $HOME
• --global installs the completion script to /etc/
• --stdout prints the completion script (default?)