Enable environment variable expansion on Repository.localfolders

Commit e51ed80e claims to add tilde and environment variable expansion
to multiple locations including Repository.localfolders. However, this
particular options seems to have been missed in that commit, and
apparently no one noticed till date

Signed-off-by: Darshit Shah <darnir@gmail.com>

offlineimap/repository/Maildir.py

@@ -71,7 +71,7 @@ class MaildirRepository(BaseRepository):
             os.utime(cur_dir, (cur_atime, os.path.getmtime(cur_dir)))
 
     def getlocalroot(self):
-        xforms = [os.path.expanduser]
+        xforms = [os.path.expanduser, os.path.expandvars]
         return self.getconf_xform('localfolders', xforms)
 
     def debug(self, msg):