auto-attach.sh and usbip not always available in the same directory

[Rasmus-Bertell]

auto-attach.sh assumes that ./usbip is always available in the same directory as itself which might not be true in more exotic distributions, leading to errors like nix-community/NixOS-WSL#594.

usbipd-win/Usbipd/WSL/auto-attach.sh

Lines 38 to 40 in cf18433

	# Use our distribution-independent build of usbip, which resides in the same directory as this very script.
	# NOTE: The working directory should have been set to the location of this script.
	LAST_ERROR=$(./usbip attach --remote="$HOST" --busid="$BUSID" 2>&1) || return 1

For context, my paths for auto-attach.sh and usbip are the following.

usbip:          /nix/store/ha3qmkizpjxxd645s4n4j8jbbxhyfj96-usbip-linux-6.6.62/bin/usbip
auto-attach.sh: /nix/store/nll5ayjp1474ax1pyq3qf52arn438dn8-auto-attach.sh