feat(display): add stdout write check in print_all_env_vars

Add a check to ensure stdout is writable before printing environment variables, preventing errors when stdout is closed. This uses a non-zero sentinel to trigger the check and mark write attempt.

Author: mattsu2020

src/uucore/src/lib/mods/display.rs

@@ -129,6 +129,7 @@ impl OsWrite for Box<dyn OsWrite> {
 /// This function handles non-UTF-8 environment variable names and values correctly by using
 /// raw bytes on Unix systems.
 pub fn print_all_env_vars<T: fmt::Display>(line_ending: T) -> io::Result<()> {
+    // Trigger stdout-closed check and mark "write attempted"; 1 is a non-zero sentinel.
     crate::check_stdout_write(1)?;
     let mut stdout = io::stdout().lock();
     for (name, value) in env::vars_os() {