test(help): Ensure consistent behavior regardless of rustup use (#15949)

### What does this PR try to resolve?

#15928 removed the fix from #13488 which breaks rust-lang/rust. This
fixes that and makes the test less brittle for the future.

### How to test and review this PR?

Author: weihanglo

tests/testsuite/cargo/help/mod.rs

@@ -6,6 +6,8 @@ use cargo_test_support::str;
 fn case() {
     snapbox::cmd::Command::cargo_ui()
         .arg("--help")
+        .env_remove("RUSTUP_HOME") // consistent behavior with/without
+        .env_remove("RUSTUP_TOOLCHAIN")
         .assert()
         .success()
         .stdout_eq(file!["stdout.term.svg"])