Auto merge of #4010 - jonhoo:better-incremental-nightly-test, r=alexcrichton

Bring test of nightly in line with tests

#4000 passes `-Zincremental` to `rustc` only on nightly, but uses a different mechanism for detecting nightly than [cargotest does](https://github.com/rust-lang/cargo/blob/9bf9bddd9297cfb5098be6146d85be551c6d4eff/tests/cargotest/lib.rs#L37). This PR brings the two in line, which should hopefully fix the build failure observed in https://github.com/rust-lang/rust/pull/41830#issuecomment-300052969.