inno: replace add_path.exe with a Pascal script While attempting to build the Inno installer, I was unable to find a copy of add_path.exe from the source site previously listed in the docs. Some quick Googling revealed that achieving this functionality in native Pascal scripts seems to be preferred these days. This commit vendors "Modify Path" (fetched from https://www.legroom.net/software/modpath) and plugs it into our Inno config file per its instructions. The existing Inno installer appears to only modify PATH for the current user (as opposed to at the system level). I've maintained this behavior with Modify Path. Although it would be trivial to change or add checkboxes to control the behavior. I'll leave this as a follow-up. Differential Revision: https://phab.mercurial-scm.org/D6060