disable tidb_read_staleness for SET_VAR

[winoros]

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

It's used during the planner's preprocess phase before we parse the SET_VAR hint.

So we disable it first.

4. What is your TiDB version? (Required)