fix: Make ui_tests test conditional on env feature

[casey]

cargo test --test ui is failing both locally on CI, I think because it expects the env feature, which is not a default feature.

This PR adds #[cfg(feature = "env")] to ui_tests, disabling the test when the feature is unavailable.

cargo test and cargo test --features env now both pass.

[epage]

on CI

Could you point me to that failure?

[casey]

Could you point me to that failure?

Whoops, I was confusing this with another error. In any case, this does make cargo test pass for me locally, when otherwise it fails, and cargo test --features env runs the UI tests and also passes.

[epage]

make test-default, which we run in CI, should catch this. The problem is clap_mangen is forcing env on when it shouldn't. I'll look into fixing that after merging this.