ci: run sysbench against 2.8 branch

This patch makes the perf_sysbench.yml workflow run against the '2.8'
branch. Unfortunately, we need to keep the logic for the '2.8' branch
in the 'master' branch workflow because scheduled workflows cannot be
run on non-default branches even they reside in them [1].

So 'matrix' is used to support workflow runs against the '2.8' branch.

[1] https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule

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