fix: Workaround Pythia8 race condition in ttbar physmon (#3965)

Until #3963 is fixed we have to stick to single threaded processing with ttbar

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## Summary by CodeRabbit

- **Bug Fixes**
	- Adjusted threading configuration to single-threaded execution to enhance stability and prevent race conditions during simulations.

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CI/physmon/workflows/physmon_trackfinding_ttbar_pu200.py

@@ -35,9 +35,10 @@
 
 
 with tempfile.TemporaryDirectory() as temp:
+    # Running with a single thread to avoid rance conditions with Pythia8, see https://github.com/acts-project/acts/issues/3963
     s = acts.examples.Sequencer(
         events=3,
-        numThreads=-1,
+        numThreads=1,  # run with single thread
         logLevel=acts.logging.INFO,
     )