CI: update for new Kas include semantics
With Kas 2.6 included YAML files need to be relative to the repository,
not the including file:
https://kas.readthedocs.io/en/latest/format-changelog.html#version-11
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>