Can't list flags given an environment

Given an environment _belongs to_ a project (e.g. `flagsmith environments --project 000`), I believe the following should work:

```
❯ flagsmith flag list --environment MySup3rC00lPr0j3ctK3y
Error: no project
Set --project, or `project` in flagsmith.json.
```

Start by tracing the `flag list --environment` command and how it resolves the project when `--project` is omitted. Reproduce the command shown in the issue, then verify that an environment belonging to a project can list its flags without reporting "no project."

索引モデルが issue の本文から書いたものです。