Reduce test-ops parallelism to avoid OOM on runner

test-ops spawns cadence_runner subprocesses per test which uses
significantly more memory than the pure-Python test-aot tests.
Limit to 4 workers to prevent runner OOM kills.

Author: aliafzal

.github/workflows/_test_cadence.yml

@@ -70,4 +70,4 @@ jobs:
         chmod +x cmake-out/backends/cadence/cadence_runner
 
         export PYTHONPATH="${PYTHONPATH:-}:$(pwd)/backends/cadence/utils/FACTO"
-        python -m pytest examples/cadence/operators/ -v -n auto
+        python -m pytest examples/cadence/operators/ -v -n 4