ci: fix email subject in perf_sysbench.yml

The `${{ github.ref_name }}` expression is always evaluated to 'master'
because the workflow is running on the 'master' branch. But it is wrong
in the case of the '2.10' branch. Now it's fixed.

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci

(cherry picked from commit b08e23f4)