`os.environ.setdefault` does not work

Creating test file
```python
# test.py
import os

os.environ.setdefault("lala", "tralala")
```

Running with `codon`
```sh
$ codon run test.py
test.py:3:1-41: error: 'EnvMap' object has no attribute 'setdefault'
$ codon build -release -exe test.py
test.py:3:1-41: error: 'EnvMap' object has no attribute 'setdefault'
```