re-bootstrapping doesn't get correct eggs sometimes
When using a generated bin/buildout to switch to another Python version like so
python2.6 bin/buildout bootstrap
the generated bin/buildout script uses the setuptools and zc.buildout eggs used by the existing bin/buildout script even if they were for a different Python version.
While the generated script uses the new python for the #! line, the eggs added to sys.path at the top aren't changed.
This is a fix-point; running the re-bootstrapping command again yields the same result.
It's not clear this is worth fixing, but it would be good if the condition were detected and caused a warning to be generated so the user could re-bootstrap in a more appropriate manner.