bug: custom `help_trigger()` not applied in `from_env()` err

Hi, i noticed custom `help_trigger()` is not applied in `from_env()` and always defaults to `--help`. If not using `--help`, you could end up with:

```
Unrecognized argument: --help

Run foo --help for more information.
```

The snippet in `from_en()`:

https://github.com/google/argh/blob/7184aab26e6434430bcbb45a9e0e8408ae830261/argh/src/lib.rs#L814

This isn't a big deal for me since I'm using my own entire help message anyways, but I just wanted to point this out.