Auto merge of #10550 - phil-opp:bindeps-doc-fix, r=weihanglo
Auto merge of #10550 - phil-opp:bindeps-doc-fix, r=weihanglo

Fix docs: Bindeps env vars are passed to build script at runtime

The environment variables are not available through the `env!` macro. Instead, they are passed by cargo when the build script is invoked.

The current test suite confirms this:

https://github.com/rust-lang/cargo/blob/fc5035d6980701c71c2ce3714e3ff239b19013b2/tests/testsuite/artifact_dep.rs#L524-L548

eminence/cargo,eminence/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo