fix: gate current_rss_kb with #[cfg(unix)] to fix Windows build

The function uses `ps` and is only called from the unix-gated
run_cli_timeout_probe, so it is dead code on Windows.

Author: psteinroe

xtask/src/leak_check.rs

@@ -335,6 +335,7 @@ fn run_leaks(pid: u32) -> anyhow::Result<String> {
     Ok(combined)
 }
 
+#[cfg(unix)]
 fn current_rss_kb(pid: u32) -> anyhow::Result<u64> {
     let output = Command::new("ps")
         .arg("-o")