fix: skip rootless reexec for help-only command paths

[immanuwell]

rootless help paths can die early right now.

nerdctl and parent commands like nerdctl system try to reexec into RootlessKit before showing help. if rootless containerd is not running, users get rootless containerd not running? instead of usage.

This skips that reexec when the command only needs to show subcommands. real runtime commands still keep the old path, so no funny business.

How to repro

1. On Linux as a non root user, make sure rootless containerd is not running.
2. Run go build -o /tmp/nerdctl ./cmd/nerdctl
3. Run /tmp/nerdctl
4. Run /tmp/nerdctl system

Before, both commands fail with rootless containerd not running?.
After, both print help. system info still follows the old path.

Kinda adjacent to #3185.

[AkihiroSuda]

Thanks