Add -DNDEBUG to benchmark builds

The benchmark library warns "Library was built as DEBUG" without NDEBUG.
This enables debug assertions and disables optimizations in the benchmark
framework, causing instrumentation overhead to show up in profiles.

script/cpp_benchmark.py

@@ -53,6 +53,7 @@ def run_benchmarks(selected_components: list[str], build_only: bool = False) ->
         # CODSPEED_ROOT_DIR is used to display relative file paths in reports.
         project_root = Path(__file__).resolve().parent.parent
         codspeed_flags = [
+            "-DNDEBUG",
             "-DCODSPEED_ENABLED",
             "-DCODSPEED_ANALYSIS",
             f'-DCODSPEED_ROOT_DIR=\\"{project_root}\\"',