Flaky test `test_deterministic_of_observed_modified_interface`

```
FAILED pymc3/tests/test_sampling.py::TestSamplePPC::test_deterministic_of_observed_modified_interface
```

This test failed here: https://github.com/pymc-devs/pymc3/runs/2423911879

And I think I have seen it before as well. We should check if it's a newly introduced bug or just needs seeding.