Fix zsh completion for commands which output is influenced by the permissions it is run with.

[kuroa-me]

This PR originates from when using nerdctl (which built its CLI with cobra), the auto-completion will not work for unprivileged users, even if they have sudo permission and are trying to tab out the command with sudo prefix.

For example, sudo nerdctl <tab> will fallback to file completion. That is because the hidden nerdctl __complete "" is run without sudo permission under the hood and fails.

This patch will enable the correct permission to be passed to the underlying hidden __complete command, thus correctly handling the case for commands which output is influenced by the permissions it is run with.

Currently, this fix is only available to zsh. And I have put it behind a switch that defaults to false.

Before

$ sudo nerdctl
completing filename
file1   file2   image1

After

$ sudo nerdctl
completing completions
apparmor    -- Manage AppArmor profiles
attach      -- Attach stdin, stdout, and stderr to a running container.
build       -- Build an image from a Dockerfile. Needs buildkitd to be runni
builder     -- Manage builds
...

$ nerdctl
completing filename
file1   file2   image1

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