Add support for variable expansion in `rules:changes:compare_to`

Paul Cacheuxrequested to merge
paul.cacheux/gitlab:paulcacheux/support-variable-expansion-compare-to into master

What does this MR do and why?

This MR adds support for variable expansion in rules:changes:compare_to. This is what was requested in #369916 (closed).

At this point this was not tested.

How to set up and validate locally

I believe this MR should be tested by reusing the test plan from the MR implementing compare_to (!90968 (merged)), but using a variable instead.

For example:

variables:
  BASE_BRANCH: main

test2:
  script: exit 0
  rules:
    - changes:
        paths:
          - README.md
        compare_to: $BASE_BRANCH

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