fix: Change SASS url variable syntax to a valid one

mentioned in commit 3e28d020

mentioned in merge request gitlab-docs!3007 (merged)

mentioned in merge request status-page!588 (merged)

mentioned in merge request gitlab!93864 (merged)

mentioned in merge request status-page!590 (merged)

mentioned in merge request status-page!591 (merged)

mentioned in merge request gitlab!95273 (closed)

mentioned in merge request gitlab-docs!3023 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3011 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!87 (merged)