Ensure rustc-echo-wrapper works with an overridden build.target-dir

Author: Nemo157

crates/cargo-test-support/src/lib.rs

@@ -1209,7 +1209,7 @@ pub trait TestEnv: Sized {
             .current_dir(&paths::root())
             .env("HOME", paths::home())
             .env("CARGO_HOME", paths::home().join(".cargo"))
-            .env("__CARGO_TEST_ROOT", paths::root())
+            .env("__CARGO_TEST_ROOT", paths::global_root())
             // Force Cargo to think it's on the stable channel for all tests, this
             // should hopefully not surprise us as we add cargo features over time and
             // cargo rides the trains.