`SetPlacementRules` fails without retrying when encountering transient network failure

## Bug Report

When handling `SetPlacementRules`, pd ran `DeleteRuleGroup` and `SaveRuleGroup` in transaction. However, when committing this transaction, RPC `Txn` encountered transient network failure. Then whole operation of `SetPlacementRules` failed without retrying.

### What did you do?

We set up the TiDB using Kubernetes and then executed the SQL below.

```sql
CREATE PLACEMENT POLICY rule1 FOLLOWERS=1;
CREATE TABLE t1 (
d date NOT NULL ,
name varchar(10) NOT NULL,
UNIQUE KEY (d,name)
) PLACEMENT POLICY="rule1";
```

### What did you expect to see?

These SQLs succeed under transient network failure, by `SetPlacementRules` retrying correctly.

### What did you see instead?

These SQLs failed.

```
ERROR 8243 (HY000): "rpc error: code = Unavailable desc = transient error"
```

### What version of PD are you using (`pd-server -V`)?

master branch