chore: explain that token env will work in snap
Description
Follow up to feat(duo): warn when running duo cli under snap... (!3451 - merged)
Part of https://gitlab.com/gitlab-org/cli/-/work_items/8372+
When Duo CLI runs with GITLAB_TOKEN env variable, it doesn't use credential hepler, the env variable takes precedence, glab forwards the env variables (tested with GITLAB_URL=http://example.com glab duo cli) and Duo CLI won't use the credential helper