Override BUNDLE_GEMFILE for `cache gems`

What does this MR do and why?

Fix failed job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/16799991

经测试，此改动应用后 cache gems Job 会成功：https://jihulab.com/gitlab-cn/gitlab/-/jobs/16808483

问题原因：环境变量 BUNDLE_GEMFILE 会影响此 Job 的行为，应该将值恢复为之前的 Gemfile。

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