Auto merge of #4000 - jonhoo:only-incremental-on-nightly, r=alexcrichton

Only pass -Zincremental to nightly rustc.

`-Z` can only be used on nightly builds; other builds complain loudly. Since incremental builds only work on nightly anyway, we should silently ignore `CARGO_INCREMENTAL` on anything but nightly. This allows users to always have `CARGO_INCREMENTAL` set without getting unexpected errors on stable/beta builds.

Fixes #3835.