pyusb: fix hardcoding libusb1 path

The line that was being replaced by the `sed` command has disappeared
from the source file.

The new `sed` script will replace the default find_library function as a
parameter to the get_backend and _load_library functions.  This means
that if a user wants to provide their own find_library implementation,
their chosen implementation will still be used.

pkgs/top-level/python-packages.nix

@@ -25940,7 +25940,7 @@ in modules // {
     postPatch = ''
       libusb=${pkgs.libusb1.out}/lib/libusb-1.0.so
       test -f $libusb || { echo "ERROR: $libusb doesn't exist, please update/fix this build expression."; exit 1; }
-      sed -i -e "s|libname = .*|libname = \"$libusb\"|" usb/backend/libusb1.py
+      sed -i -e "s|find_library=None|find_library=lambda _:\"$libusb\"|" usb/backend/libusb1.py
     '';
 
     # No tests included