libstd: Pass-through PATH in test_override_env

In some build environments (such as Nix builds), `env` can only be found
in the explicitly-provided PATH, not in default places such as /bin or
/usr/bin. So we need to pass-through PATH when spawning the `env`
sub-process.

Fixes rust-lang#17617

Author: wizeman

src/libstd/io/process.rs

@@ -956,7 +956,22 @@ mod tests {
     })
 
     iotest!(fn test_override_env() {
-        let new_env = vec![("RUN_TEST_NEW_ENV", "123")];
+        use os;
+        let mut new_env = vec![("RUN_TEST_NEW_ENV", "123")];
+
+        // In some build environments (such as Nix builds), `env` can only be
+        // found in the explicitly-provided PATH env variable, not in default
+        // places such as /bin or /usr/bin. So we need to pass through PATH to
+        // our sub-process.
+        let path_val: String;
+        match os::getenv("PATH") {
+            None => {}
+            Some(val) => {
+                path_val = val;
+                new_env.push(("PATH", path_val.as_slice()))
+            }
+        }
+
         let prog = env_cmd().env_set_all(new_env.as_slice()).spawn().unwrap();
         let result = prog.wait_with_output().unwrap();
         let output = String::from_utf8_lossy(result.output.as_slice()).into_string();