The disk_backed_refuses_preexisting_symlink_at_jail_tmp test calls
std::os::unix::fs::symlink directly, which doesn't exist on Windows
targets. Without a cfg gate, `cargo check --tests` fails on Windows
with E0433. Other symlink call sites in this crate (php_executor,
bun_executor, rust_executor, etc.) already follow this pattern.
Fixes WIN-1972
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>