Disable virtualenv warning in bash on Windows

This is for IPython installed in a virtual environment on Windows. When I run IPython in bash I get the warning:
```
WARNING: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
```

Four tests fail in bash:
```
Test we're not loading modules on startup that we shouldn't. ... ERROR
IPython.core.tests.test_run.TestMagicRunSimple.test_obj_del ... ERROR
Test that `__file__` is set when running `ipython file.ipy` ... ERROR
Test that `__file__` is set when running `ipython file.py` ... ERROR
```

Nitpick: now that there's an environment variable lookup and a function call in here, let's move it outside the loop and use a local variable for the normalised virtualenv path.