ci: Don't reuse EE fixtures in as-if-foss jobs

What does this MR do and why?

!117046 (diffs, comment 1366421692) introduced a regression where as-if-foss jobs would download fixtures artifacts from retrieve-frontend-fixtures, which are stored under tmp/tests/frontend/fixtures-ee instead of tmp/tests/frontend/fixtures.

This would result in failures such as https://gitlab.com/gitlab-org/gitlab/-/jobs/4171525868.

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