Add REVIEW_SLUG to environment for review apps scripts

We need to add $REVIEW_SLUG to the URL in the scripts that deploy the review apps as well. It was added in some spots in the related MR, so the deployment environment now works properly in GitLab, but while watching a deployment just now (https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3203038064), I saw that the script still didn't work. It deployed straight to https://main.docs.gitlab-review.app, NOT https://main-ee-101347.docs.gitlab-review.app, so the $REVIEW_SLUG is missing from the script.

• Related to !3198 (merged)
• Related to #735 (closed)