Eggs installed by Distribute in the system Python precede eggs selected by our buildout branch

As reported by Stuart Bishop in a comment to bug 389065:

$ bin/py
Python 2.5.5 (r255:77872, Jan 31 2010, 21:34:29)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pytz
>>> pytz.__version__
'2009e'
>>> import sys
>>> sys.path
['', '/usr/lib/python2.5/site-packages/pytz-2009e-py2.5.egg', '/home/stub/lp/trivial/parts/scripts', '/usr/lib/python2.5', '/usr/lib/python2.5/plat-linux2', '/usr/lib/python2.5/lib-tk', '/usr/lib/python2.5/lib-dynload', '/home/stub/lp/lp-sourcedeps/eggs/setuptools-0.6c11-py2.5.egg', '/home/stub/lp/trivial/lib', '/home/stub/lp/lp-sourcedeps/eggs/windmill-1.3beta3_lp_r1440-py2.5.egg', '/home/stub/lp/lp-sourcedeps/eggs/funkload-1.10.0-py2.5.egg', '/home/stub/lp/lp-sourcedeps/eggs/zc.zservertracelog-1.1.5-py2.5.egg', '/home/stub/lp/lp-sourcedeps/eggs/zope.server-3.6.1-py2.5.egg'