On another linux system I might expect to find a .profile file, or something in /etc used by default. I'm unable to determine what sets the system path for processes not running in a shell.
It appears to be defined in /init.rc, at least on my device:
export PATH /sbin:/system/sbin:/system/bin:/system/xbin
I don't think you can edit this file directly though, even with root access, because it is part of the read-only boot image, not the /system partition. If you want to edit it then I guess you would have to unpack, edit and re-pack the boot image as described here:
I haven't tried this though.