env: handle the error properly

instead of:
env: unknown error: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }

tests/by-util/test_env.rs

@@ -80,6 +80,15 @@ fn test_env_version() {
         .stdout_contains(util_name!());
 }
 
+#[test]
+fn test_env_permissions() {
+    new_ucmd!()
+        .arg(".")
+        .fails()
+        .code_is(126)
+        .stderr_is("env: '.': Permission denied\n");
+}
+
 #[test]
 fn test_echo() {
     #[cfg(target_os = "windows")]