`humility --list-targets` fails confusingly when `HUMILITY_TARGET` is set

[hawkw]

eliza@jeeves ~ $ export HUMILITY_TARGET='mb-3-sp'
eliza@jeeves ~ $ humility --list-targets
humility: WARNING: archive in environment variable overriding archive in environment file
humility failed: subcommand expected (--help to list)
eliza@jeeves ~ $ unset HUMILITY_TARGET
eliza@jeeves ~ $ humility --list-targets
TARGET          DESCRIPTION
ee-1-sp         EE Station 1 SP
mb-0-sp         Minibar 0's target SP
mb-1-sp         Minibar 1's target SP
mb-2-sp         Minibar 2's target SP
mb-3-sp         Minibar 3's target SP
eliza@jeeves ~ $

I get why it does that: clap interprets the presence of the environment variable as being equivalent to humility --list-targets --target mb-3-sp, which is a nonsensical command line --- the presence of the --target means that it wants a subcommand.

But it feels very weird to me that the same same command line fails or succeeds depending on the env var. I also think the "subcommand expected" error is quite confusing here if you aren't already uncomfortably familiar with how clap's support for environment variables work --- one must understand that it treats HUMILITY_TARGET being set as exactly the same as --target being passed in order to interpret the error message.

Perhaps the best solution here would be to change --list-targets to be list-targets (i.e. a subcommand) instead. It being a flag is a bit odd anyway, since it doesn't make sense to pass --list-targets along with any other subcommand anyway. Weirdly, that does work, which also kind of doesn't make sense:

eliza@jeeves ~ $ humility --list-targets tasks
TARGET          DESCRIPTION
ee-1-sp         EE Station 1 SP
mb-0-sp         Minibar 0's target SP
mb-1-sp         Minibar 1's target SP
mb-2-sp         Minibar 2's target SP
mb-3-sp         Minibar 3's target SP
eliza@jeeves ~ $

it listed targets instead of doing humility tasks...I would have expected this to fail because humility tasks expects a probe, archive, or IP. Which, if you pass that, does make --list-targets break:

eliza@jeeves ~ $ humility --list-targets --target 'mb-3-sp' tasks
error: The argument '--list-targets' cannot be used with:
    --probe <PROBE>
    --dump <DUMP>
    --ip <IP>
    --target <TARGET>

USAGE:
    humility --list-targets --target <TARGET> --archive <ARCHIVE> --environment <ENVIRONMENT>

For more information try --help
eliza@jeeves ~ $

This isn't as bad a UX issue as the confusing error message when run with the env set, but it is a bit odd.