Don't deny warnings in UI tests

[jswrenn]

As of #548, cargo.sh forwards RUSTFLAGS from the ambient environment to cargo. In CI, one such set flag is -Dwarnings, which elevates all warnings to errors. This impacts our UI tests: things which are, really, warnings, appear to be errors. Since the purpose of our UI tests is to confirm that compile errors are produced as expected on invalid code, this could lull us into a false sense of security if an "error" is really a warning.

Less pressingly, it currently means that all local trybuild runs during development need to be preceded by RUSTFLAGS="-Dwarnings" in order for the .stderr files to match in CI.

We need to prevent RUSTFLAGS from modifying lint levels when running UI tests. Ideally, this takes the form of porting to the ui-test crate, which I believe doesn't have this problem. See #187.

[joshlf]

@jswrenn Was this fixed by #567?