Fix run-make-support to respect per-stage cargo

Author: jieyouxu

src/tools/run-make-support/src/external_deps/cargo.rs

@@ -1,7 +1,8 @@
 use crate::command::Command;
 use crate::env_var;
 
-/// Returns a command that can be used to invoke Cargo.
+/// Returns a command that can be used to invoke cargo. The cargo is provided by compiletest
+/// through the `CARGO` env var.
 pub fn cargo() -> Command {
-    Command::new(env_var("BOOTSTRAP_CARGO"))
+    Command::new(env_var("CARGO"))
 }