Use ENV to determine Cygwin location

Cygwin uses the environment variable HOMEPATH to store the portion of the path to the user's home directory that comes after the drive letter. So if Cygwin is installed in D:\apps\cygwin, HOMEPATH will be something like \apps\cygwin\home\username. Therefore, assuming the user hasn't modified HOMEPATH for some reason, we can derive the correct path to Cygwin.