test: redact host target when comparing CARGO_ENV path

This was found when updating git submodule in rust-lang/rust

```
---- test::cargo_test_env stdout ----
running `/Users/weihanglo/dev/rust/build/aarch64-apple-darwin/stage2-tools/aarch64-apple-darwin/release/cargo test --lib -- --nocapture`
thread 'test::cargo_test_env' panicked at tests/testsuite/test.rs:3904:10:

test failed running `/Users/weihanglo/dev/rust/build/aarch64-apple-darwin/stage2-tools/aarch64-apple-darwin/release/cargo test --lib -- --nocapture`
error: expected to find:
/Users/weihanglo/dev/rust/build/aarch64-apple-darwin/stage2-tools/aarch64-apple-darwin/release/cargo

did not find in output:
[COMPILING] foo v0.5.0 ([ROOT]/foo)
[FINISHED] `test` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
[RUNNING] unittests src/lib.rs (target/debug/deps/foo-[HASH])
/Users/weihanglo/dev/rust/build/[HOST_TARGET]/stage2-tools/[HOST_TARGET]/release/cargo
```

tests/testsuite/test.rs

@@ -3870,6 +3870,7 @@ fn doctest_and_registry() {
 
 #[cargo_test]
 fn cargo_test_env() {
+    let rustc_host = rustc_host();
     let src = format!(
         r#"
         #![crate_type = "rlib"]
@@ -3896,6 +3897,7 @@ fn cargo_test_env() {
             .with_extension("")
             .to_str()
             .unwrap()
+            .replace(rustc_host, "[HOST_TARGET]")
     );
     p.cargo("test --lib -- --nocapture")
         .with_stderr_contains(cargo)
@@ -3911,7 +3913,14 @@ test env_test ... ok
         .unwrap()
         .canonicalize()
         .unwrap();
-    let stderr_rustc = format!("{}[EXE]", rustc.with_extension("").to_str().unwrap());
+    let stderr_rustc = format!(
+        "{}[EXE]",
+        rustc
+            .with_extension("")
+            .to_str()
+            .unwrap()
+            .replace(rustc_host, "[HOST_TARGET]")
+    );
     p.cargo("test --lib -- --nocapture")
         // we use rustc since $CARGO is only used if it points to a path that exists
         .env(cargo::CARGO_ENV, rustc)