docs about cargo-provided environment variables could be clearer that they're compile-time only

This section of the docs:
https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates
(https://github.com/ehuss/cargo/blob/master/src/doc/src/reference/environment-variables.md#environment-variables-cargo-sets-for-crates)

says:

Cargo exposes these environment variables to your crate when it is compiled. Note that this applies for running binaries with cargo run and cargo test as well.

I think what this means to say is something like: "Cargo exposes these environment variables to your crate when any of its targets is compiled, including when compiling binaries that you might later run with cargo run and cargo test. [These environment variables are not provided at runtime.]"

It's easy to misread what's there as: "Cargo exposes these environment variables to your crate when it is compiled, and when running binaries with cargo run and cargo test as well."

The difference is whether the variables are available at runtime, which as far as I can tell they aren't.

Start in src/doc/src/reference/environment-variables.md at the “Environment variables Cargo sets for crates” section, and compare it with the published documentation. Clarify that the variables are available while targets are compiled, including for cargo run and cargo test, rather than at runtime. Done when the wording cannot be read as promising runtime environment variables.