ci: reduce parallel benchmark jobs from 16 to 12

Lower max-parallel in benchmark workflow to reduce resource contention and avoid intermittent failures.

Author: mattsu2020

.github/workflows/benchmarks.yml

@@ -26,7 +26,7 @@ jobs:
       CARGO_INCREMENTAL: 0
     strategy:
       fail-fast: false
-      max-parallel: 16
+      max-parallel: 12
       matrix:
         type: [simulation, memory]
         package: [