setup: configure py2exe config via environment variables The Inno Setup and WiX installers ship a different set of packages with py2exe builds. And there are multiple WiX installer variants (e.g. TortoiseHG). Since there are multiple variants of py2exe configs and they can be defined by entities not in our repository, let's provide a mechanism for setup.py to supplement behavior via environment variables. This is slighly less hacky than a setup.cfg file IMO since the caller doesn't need to worry about mutating global state of the source directory. Differential Revision: https://phab.mercurial-scm.org/D6092