Can't install packages using pip in Virtualenv on remote using Fabric - deployment

Question

I have being using fabric to deploy an app with virtualenv. I was using fabric 1.4 and upgraded to 1.5.1 last week. My script stopped working.
It can't install the requirements. It seems it's not activating the virtualenv. In my code, I have:
with cd('%(path)s' % env):
with prefix('source bin/activate'):
run('pip install -U distribute')
I'm getting a permission denied error: error: could not delete '/usr/local/lib/python2.7/dist-packages/pkg_resources.py': Permission denied
The command being executed is:
Executed: /bin/bash -l -c "cd /var/www/myproject && source bin/activate && export PATH=\"\\$PATH:\\"/var/www/myproject\\" \" && pip install -U distribute"
If I ssh to the remote machine and run cd /var/www/myproject && source bin/activate && pip install -U distribute, it works just fine.
Why is my fabric script not working?
Thanks in advance

Answer 1

Instead of the serial approach with..
source bin/activate
pip install -U distribute
..directly use the pip executable of the virtualenv:
myenv/bin/pip install -U distribute

Answer 2

Although not exactly a solution, fabtools has a number of functions related to virtualenvs that are very handy. They do (almost) all of the hard work for you, and are probably worth using to check it isn't something else going wrong.
# Cut (and modified) from the fabtools documentation
from fabric.api import *
from fabtools import require
import fabtools
#task
def setup():
# Require a Python package
with fabtools.python.virtualenv('/home/myuser/env'):
require.python.package('pyramid')