This is passed when calling `install(['zc.buildout'])` to resolve the
dists of the currently running buildout and its dependencies, as they
may not actually correspond the version of the running interpreter.

This is needed to support new slapos.rebootstrap: after compiling the
new Python interpreter, we use it to run buildout bootstrap using the
same paths to zc.buildout and dependencies as in the initial Python.
This installs the needed dists for the new Python version and creates
a new bin/buildout for the new Python interpreter.