fix(stylelint): css variable regex

mentioned in commit 851b7687

mentioned in merge request gitlab-org/technical-writing/docs-gitlab-com!676 (merged)

mentioned in merge request gitlab-svgs!1168 (merged)

mentioned in merge request gitlab-svgs!1169 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab-svgs!1170 (closed)

mentioned in merge request gitlab!191470 (merged)

mentioned in merge request gitlab!191777 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4433 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1731 (merged)