scripts: zspdx: fix writing custom license IDs

The builtin list function `.sort()` sorts the list in-place and returns
None. As this is an invalid type for iteration, use the builtin `sorted`
function, which returns a sorted copy of the list, which we can iterate
over.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>
(cherry picked from commit 06aae610)