ci: fix attribute interpolation in preview-head.pug

In !805 (merged) we introduced a few changes to the Pug template that generates the preview-head.html file for Storybook. Specifically, we put the application.css URL into a variable in order to be able to use this URL in several places in the template.

One of the use cases is for generating the <link> tag to import GitLab's CSS when running visual integration tests, but the interpolation currently isn't evaluated and the rendered link looks like this:

<link type="text/css" rel="stylesheet" media="all" href="#{gitlabCssLink}"/>

Which broke the visual_gitlab_integration CI job, notice how all tests cases failed here: https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/314928332

This MR ensures that the interpolation is properly evaluated so that the the link renders to:

<link type="text/css" rel="stylesheet" media="all" href="https://gitlab-org.gitlab.io/gitlab/application.css"/>

We now have only 9 failed tests, which is expected since some components' styles aren't up-to-date yet: https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/314980485