test_env: Try to execute an empty file instead of .

For some unclear reason, Android _now_ sometimes returns an
IsADirectory error, instead of PermissionDenied, when trying
to execute `.`.

Since this test really wants to test PermissionDenied, we try
to execute a file in the fixture instead, that doesn't have
exec permission.

Also, limit this test to Unix.

Fixes part of #7542.

tests/by-util/test_env.rs

@@ -81,11 +81,13 @@ fn test_env_version() {
 }
 
 #[test]
+#[cfg(unix)]
 fn test_env_permissions() {
+    // Try to execute `empty` in test fixture, that does not have exec permission.
     new_ucmd!()
-        .arg(".")
+        .arg("./empty")
         .fails_with_code(126)
-        .stderr_is("env: '.': Permission denied\n");
+        .stderr_is("env: './empty': Permission denied\n");
 }
 
 #[test]