Rollup merge of rust-lang#47718 - malbarbo:env-home-dir, r=nikomatsakis

Make run-pass/env-home-dir.rs test more robust

Remove the assumption that home_dir always returns Some.

This allows the test to be executed with [cross](https://github.com/japaric/cross).

src/test/run-pass/env-home-dir.rs

@@ -27,7 +27,10 @@ fn main() {
     if cfg!(target_os = "android") {
         assert!(home_dir().is_none());
     } else {
-        assert!(home_dir().is_some());
+        // When HOME is not set, some platforms return `None`,
+        // but others return `Some` with a default.
+        // Just check that it is not "/home/MountainView".
+        assert_ne!(home_dir(), Some(PathBuf::from("/home/MountainView")));
     }
 }