unclosetx: false positive detection with errgroup about zagane

[tenntenn]

more simple sample code:

func f5(ctx context.Context, client *spanner.Client) error {
	tx := client.ReadOnlyTransaction() // OK
	defer tx.Close()

	func() error {
		_ = tx // use tx
		return nil
	}()

	return nil
}

[tenntenn]

I think this issue has two bugs.
First bug is false positive of tx := client.ReadOnlyTransaction().
Second bug is false positive of _ = tx // use tx.
Actually the below code zagane reports false positive in only case (2).

func() error {
	func(tx *spanner.ReadOnlyTransaction) {}(tx) // (1) OK
	_ = tx // (2) NG
	return nil
}()