fix(ci): use OPENAI_KEY instead of OPENAI.KEY in pr-review workflow

[qin-ctx]

Description

修复 pr-review workflow 中 OPENAI.KEY 应为 OPENAI_KEY（下划线），导致 workflow startup_failure 的问题。

Related Issue

Fixes startup_failure in PR Review workflow

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

• 将 OPENAI.KEY 改为 OPENAI_KEY，与 pr-agent 官方文档一致

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