Fix sticky filters not working on vulnerability report

What does this MR do and why?

This MR fixes a bug that was introduced in Overridable $header-height SCSS variable (!77040 - merged) that broke a sticky header on the vulnerability report.

Before	After

You can see this issue in production here: https://gitlab.com/gitlab-org/gitlab/-/security/vulnerability_report

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #327867 (closed)