tests(env): increase signal delivery delay on macOS to fix flaky CI

The test_env_arg_ignore_signal_valid_signals test was failing on macOS
CI because 100ms was not always enough for the process to terminate
after receiving a fatal signal. Increase the delay to 500ms on macOS
only, keeping 100ms on other platforms.

Fixes flaky failure seen in: https://github.com/uutils/coreutils/actions/runs/24442074142/job/71409227191

tests/by-util/test_env.rs

@@ -52,6 +52,10 @@ impl Target {
             .spawn()
             .expect("failed to send signal")
             .wait();
+        // macOS CI needs a longer delay for process teardown after signal delivery
+        #[cfg(target_os = "macos")]
+        self.child.delay(500);
+        #[cfg(not(target_os = "macos"))]
         self.child.delay(100);
     }
     fn is_alive(&mut self) -> bool {