Fix CARGO_INCREMENTAL semantics to be intuitive

Fix CARGO_INCREMENTAL semantics to be intuitive

Previously, the mere presence of a CARGO_INCREMENTAL variable in the
environment caused incremental compilation to happen. This has the very
unintuitive effect that `CARGO_INCREMENTAL=0` and even
`CARGO_INCREMENTAL=` mean incremental compilation is *on*.

This change brings the semantics in line with how they are defined in
the tests (cf. tests/build.rs:45), and in public-facing documentation
(https://internals.rust-lang.org/t/incremental-compilation-beta/4721).