`simulate.py` scans return only 1 event

### Environment: (where does this bug occur, have you tried other environments)
- Which branch (often `main` for latest released): `main`

### Steps to reproduce: (give a step by step account of how to trigger the bug)
1. `simulate.py -t 6 -k 5 -n 30` (eta scan, used in CI)

Likely other tests (set with `-t` option) fail as well. `simulate.py` generates a Geant4 macro file then runs it with `npsim`; the tests that now (likely) fail have the following pattern:

```
/control/verbose 2
/run/initialize
/gps/verbose 2
/gps/particle pi+
/gps/number 1
/gps/position 0 0 0 cm

# eta scan, with varied azimuth for each eta bin

# eta=1.6 theta=22.828754832963863 deg
/gps/direction 0.2679262666315343 -0.2806111036063496 0.9216685544064713
/gps/ene/mono 4.862377599175745 GeV
/run/beamOn 1
/gps/direction 0.35670337364908533 -0.15260989170314782 0.9216685544064713
/gps/ene/mono 4.862377599175745 GeV
/run/beamOn 1
/gps/direction 0.355244674481362 -0.15597530917076796 0.9216685544064713
/gps/ene/mono 4.862377599175745 GeV
/run/beamOn 1
/gps/direction -0.3269047091825161 -0.2089506806224379 0.9216685544064713
/gps/ene/mono 4.862377599175745 GeV
/run/beamOn 1
/gps/direction 0.1315721232832751 0.3649874685424366 0.9216685544064713
/gps/ene/mono 4.862377599175745 GeV
/run/beamOn 1

...
```

### Expected Result: (what do you expect when you execute the steps above)
An output file with many events, and `stdout` from `npsim` includes (irrelevant lines omitted):
```
ParticleHandler INFO +++ Event 0 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 1 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 2 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 3 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 4 Begin event action. Access event related information.
...
```

### Actual Result: (what do you get when you execute the steps above)

An output file with 1 event, and the corresponding `stdout` is repeated for event `0`:
```
ParticleHandler INFO +++ Event 0 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 0 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 0 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 0 Begin event action. Access event related information.
ParticleHandler INFO +++ Event 0 Begin event action. Access event related information.
...
```

See [example CI log](https://github.com/eic/drich-dev/actions/runs/6029699062/job/16361572743?pr=98)