Gate home_dir import on non-Android

Fix the remaining Android build warning by only importing dirs::home_dir when it is used.

This keeps the desktop default unchanged and avoids an unused_imports warning under cargo-ndk.

codex-rs/utils/home-dir/src/lib.rs

@@ -1,3 +1,4 @@
+#[cfg(not(target_os = "android"))]
 use dirs::home_dir;
 use std::path::PathBuf;