setup: properly install build_hgextindex for py2exe builds Because the hgbuild class has a private copy of build.sub_commands, modifying build.sub_commands from this code effectively resulted in a no-op. Registering the sub-command on hgbuild actually results in the sub-command running when building Mercurial. Differential Revision: https://phab.mercurial-scm.org/D6093