Disable parallel runs for cng by default

What does this MR do and why?

As part of Run experiment with running 'test-on-cng' in me... (gitlab-org/quality/quality-engineering/team-tasks#3049 - closed) after monitoring test performance for a while, it seems that there are just a little bit too many random failures here and there. Because the plan is to enable test-on-cng in a blocking capacity, stability has to take slightly higher priority to runtime.

After Improve E2E test framework for test isolation t... (gitlab-org/quality/quality-engineering&99) is completed and potentially some additional monitoring is added to reliably see what could affect flakiness when running in parallel, this could be revisited.

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