Set blank feature flag env var in GDK tests

What does this MR do and why?

This is a workaround to avoid enabling feature flags in GDK e2e tests and causing them to block MRs (see !137890 (comment 1681217315))

Tested in !138876 (closed)

As a followup we should add a job that does toggle the feature flag (but allow it to fail), and keep the feature flag in its default state the original blocking job. gitlab-org/quality/quality-engineering/team-tasks#2232 (closed)

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