fix(nsjail): gate unix-symlink test behind cfg(unix) for Windows build (#9280)

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>

backend/windmill-worker/src/common.rs

@@ -1234,6 +1234,7 @@ mod nsjail_tmp_mount_tests {
     /// `job_dir` before the resolver runs), the resolver must refuse the
     /// bind mount and fall back to tmpfs — never bind-mount the symlink
     /// target into the sandbox as /tmp.
+    #[cfg(unix)]
     #[tokio::test]
     async fn disk_backed_refuses_preexisting_symlink_at_jail_tmp() {
         let tmp = tempfile::tempdir().expect("tempdir");