Set DRY_RUN to false if not set

What does this MR do and why?

!112690 (merged) introduced a bug when the DRY_RUN env variable isn't set (e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/3972903306):

$ scripts/review_apps/automated_cleanup.rb --dry-run="${DRY_RUN}" || (scripts/slack review-apps-monitoring "☠️ \`${CI_JOB_NAME}\` failed! ☠️ See ${CI_JOB_URL} - <https://gitlab.com/gitlab-org/quality/engineering-productivity/team/-/blob/main/runbooks/review-apps.md#review-cleanup-job-failed|📗 RUNBOOK 📕>" warning "GitLab Bot" && exit 1);
scripts/review_apps/automated_cleanup.rb:295:in `<main>': invalid argument: --dry-run= (OptionParser::InvalidArgument)

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