rules:changes:compare_to support to variable expansion

#293645 (closed) introduced compare_to property to rules:change, however this new property does not support variable expansion.

Supporting variable expansion would be useful to allow job definitions like:

    - changes:
        paths:
          - README.md
        compare_to: "$CI_DEFAULT_BRANCH"

Or

    - if: $CI_COMMIT_BEFORE_SHA != "0000000000000000000000000000000000000000"
      changes:
        paths:
          - README.md
        compare_to: "$CI_COMMIT_BEFORE_SHA"