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.

Author: phil-opp

src/doc/src/reference/unstable.md

@@ -886,7 +886,7 @@ bar = { artifact = "cdylib", version = "1.0", target = "wasm32-unknown-unknown"
 
 ```rust
 fn main() {
-  wasm::run_file(env!("CARGO_CDYLIB_FILE_BAR"));
+  wasm::run_file(std::env::var("CARGO_CDYLIB_FILE_BAR").unwrap());
 }
 ```