Expand variables in rules:changes

[PigeonF]

This enables common parallel:matrix configurations like

matrix:
  image: alpine
  parallel:
    matrix:
      - SUBDIR:
          - foo
          - bar
  rules:
    - changes:
        paths:
          - "${SUBDIR}/**/*"
  script:
    - echo "Job is running"

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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