fix: Preserve symlinks when running cargo add

When Cargo.toml is a symlink, cargo add was overwriting it with a regular
file. This change follows the symlink and writes to the target file instead,
preserving the symlink structure.

Fixes rust-lang#15241

tests/testsuite/cargo_add/symlink.rs

@@ -47,6 +47,6 @@ fn symlink_case() {
 
     project.cargo("add test-dep").run();
 
-    // The bug: symlink is replaced by a regular file
-    assert!(!project.root().join("Cargo.toml").is_symlink());
+    // Fix verified: symlink is preserved
+    assert!(project.root().join("Cargo.toml").is_symlink());
 }