CI: Incorrect variable substitution in job:environment:url

variables: &variables
  REVIEWAPPS_SUBDOMAIN: "$CI_ENVIRONMENT_SLUG"
  REVIEWAPPS_DOMAIN: some-url-here.io

job:
  environment:
    name: review/$CI_COMMIT_REF_NAME
    url: https://$REVIEWAPPS_SUBDOMAIN.$REVIEWAPPS_DOMAIN/
    on_stop: reviewapp-delete

This gets me the environment URL: https://%24ci_environment_slug.some-url-here.io/

I can imagine understand why this doesn't work... But it's a little counter-intuitive given he fact the command below returns the expected value, e.g. review-feature-ld-z3ageq some-url-here.io.

script:
- echo "$REVIEWAPPS_SUBDOMAIN" "$REVIEWAPPS_DOMAIN"

~"CI/CD" cc @grzesiek @markpundsack