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).

rustyhorde/cargo,vojtechkral/cargo,rust-lang/cargo,achanda/cargo,nipunn1313/cargo,nipunn1313/cargo,tsurai/cargo,tshepang/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,durka/cargo,tshepang/cargo,tshepang/cargo,durka/cargo,achanda/cargo,rust-lang/cargo,eminence/cargo,achanda/cargo,rust-lang/cargo,tsurai/cargo,rustyhorde/cargo,tshepang/cargo,nipunn1313/cargo,rust-lang/cargo,vojtechkral/cargo,vojtechkral/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,tsurai/cargo,achanda/cargo,durka/cargo,durka/cargo