Rename usages of `spec` in benchmark_spec.py
Rename usages of `spec` in benchmark_spec.py

PiperOrigin-RevId: 360347796

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker