FIX: EggInst.run uses wrong python

When:

(line 118 of egginst/main.py)
```
        if self.prefix != abspath(sys.prefix):
            scripts.executable = get_executable(self.prefix)
```

The scripts.executable is changed to use the python found in the prefix directory. The run method was still using sys.executable meaning that post_egginst and pre_egguninst scripts where run against the System venv and not the User one.