xdist adds current directory to sys.path

With 1.26.1.
```python
import sys

def test_path():
    assert "" not in sys.path
```
```
$ pytest test_path.py
<OK>
$ pytest test_path.py -n 1
E     AssertionError....
```
Obviously this can cause problems/change behaviour depending on where you are running the tests from.

Possibly related to #376?