Add mt bench run to CI

Author: igchor

.github/workflows/benchmarks.yml

@@ -57,3 +57,7 @@ jobs:
           fi
           cat $LOG
           echo "[  PASSED  ] The CI benchmark job PASSED."
+
+      - name: Run MT benchmarks
+        working-directory: ${{github.workspace}}/build
+        run: ./benchmark/multithread_bench