Check RUSTC_WRAPPER instead of CARGO_RUSTC_WRAPPER.

Similar to dtolnay/anyhow#248, and as suggested [here], check `RUSTC_WRAPPER`
instead of `CARGO_RUSTC_WRAPPER`.

[here]: #544 (comment)

build.rs

@@ -117,7 +117,7 @@ fn main() {
     println!("cargo:rerun-if-env-changed=CARGO_ENCODED_RUSTFLAGS");
     println!("cargo:rerun-if-env-changed=RUSTC");
     println!("cargo:rerun-if-env-changed=TARGET");
-    println!("cargo:rerun-if-env-changed=CARGO_RUSTC_WRAPPER");
+    println!("cargo:rerun-if-env-changed=RUSTC_WRAPPER");
     println!("cargo:rerun-if-env-changed=PROFILE");
 }
 
@@ -199,7 +199,7 @@ fn can_compile<T: AsRef<str>>(test: T) -> bool {
     let rustc = var("RUSTC").unwrap();
     let target = var("TARGET").unwrap();
 
-    let mut cmd = if let Ok(wrapper) = var("CARGO_RUSTC_WRAPPER") {
+    let mut cmd = if let Ok(wrapper) = var("RUSTC_WRAPPER") {
         let mut cmd = std::process::Command::new(wrapper);
         // The wrapper's first argument is supposed to be the path to rustc.
         cmd.arg(rustc);