Don't inherit global variables when starting review-apps pipeline

What does this MR do and why?

We have opened #387183 (closed) to understand why we couldn't override GIT_STRATEGY variable when in a downstream pipeline (e.g. review-apps). It turns out that all global variables will be passed to a child pipeline with the highest precedence possible, so all of the global variables are not overridable in a child pipeline (see source).

This MR changes this, as we want to still be able to override variables from child pipelines.

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