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

[phil-opp]

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:

cargo/tests/testsuite/artifact_dep.rs

Lines 524 to 548 in fc5035d

	fn main() {
	let baz: std::path::PathBuf = std::env::var("CARGO_BIN_FILE_BAR_baz").expect("CARGO_BIN_FILE_BAR_baz").into();
	println!("{}", baz.display());
	assert!(&baz.is_file());
	let lib: std::path::PathBuf = std::env::var("CARGO_STATICLIB_FILE_BAR_bar").expect("CARGO_STATICLIB_FILE_BAR_bar").into();
	println!("{}", lib.display());
	assert!(&lib.is_file());
	let lib: std::path::PathBuf = std::env::var("CARGO_CDYLIB_FILE_BAR_bar").expect("CARGO_CDYLIB_FILE_BAR_bar").into();
	println!("{}", lib.display());
	assert!(&lib.is_file());
	let dir: std::path::PathBuf = std::env::var("CARGO_BIN_DIR_BAR").expect("CARGO_BIN_DIR_BAR").into();
	println!("{}", dir.display());
	assert!(dir.is_dir());
	let bar: std::path::PathBuf = std::env::var("CARGO_BIN_FILE_BAR").expect("CARGO_BIN_FILE_BAR").into();
	println!("{}", bar.display());
	assert!(&bar.is_file());
	let bar2: std::path::PathBuf = std::env::var("CARGO_BIN_FILE_BAR_bar").expect("CARGO_BIN_FILE_BAR_bar").into();
	println!("{}", bar2.display());
	assert_eq!(bar, bar2);
	}

[rust-highfive]

r? @ehuss

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[weihanglo]

Looks great to me. Thanks!

[weihanglo]

@bors r+

[bors]

📌 Commit 90f6a99 has been approved by weihanglo

[bors]

⌛ Testing commit 90f6a99 with merge 4777c22...

[bors]

☀️ Test successful - checks-actions
Approved by: weihanglo
Pushing 4777c22 to master...