message: Add note about `cargo:rustc-env` variables set at runtime.
diff: @@ -192,6 +192,12 @@ unique identifier of a continuous integration server.
  See also the [environment variables automatically included by
  Cargo][env-cargo].
  +> **Note**: These environment variables are also set when running an
  +> executable with `cargo run` or `cargo test`. However, this usage is
  +> discouraged since it ties the executable to Cargo's execution environment.
  +> Normally, these environment variables should only be checked at compile-time
  +> with the `env!` macro.
  +
  [env-macro]: ../../std/macro.env.html
  [env-cargo]: environment-variables.md#environment-variables-cargo-sets-for-crates
  