cmd/cue: help cmd $cmd does not exit non-zero for unknown command

[myitcv]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.0.0-20230821113919-220b44e973ef

go version go1.20.6
      -buildmode exe
       -compiler gc
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
             vcs git
    vcs.revision 220b44e973eff9d41f2067499f20a12b6a0a6f29
        vcs.time 2023-08-21T11:39:19Z
    vcs.modified false

Does this issue reproduce with the latest release?

Yes

What did you do?

exec cue help cmd x
stdout 'long desc'

! exec cue help cmd doesnotexist
stderr 'unknown command .* doesnotexist'

-- x_tool.cue --
package x

import "tool/cli"

command: x: cli.Print & {
	$long: "long desc"
	text: "test"
}

What did you expect to see?

Passing test (modulo the correct expectation on stderr)

What did you see instead?

# known command (0.016s)
# unknown command (0.012s)
> ! exec cue help cmd doesnotexist
[stdout]
cmd executes the named command for each of the named instances.

...

Run "cue help commands" for more details on tasks and commands.

Usage:
  cue cmd <name> [inputs] [flags]
  cue cmd [command]

Available Commands:
  x

Flags:
  -t, --inject stringArray   set the value of a tagged field
  -T, --inject-vars          inject system variables in tags (default true)

Global Flags:
  -E, --all-errors   print all available errors
  -i, --ignore       proceed in the presence of errors
  -s, --simplify     simplify output
      --strict       report errors for lossy mappings
      --trace        trace computation
  -v, --verbose      print information about progress

Use "cue cmd [command] --help" for more information about a command.
FAIL: /tmp/testscript1425870910/repro.txtar/script.txtar:6: unexpected command success

i.e. the requesting for help for an unknown command resulted in a zero exit code.