[CMAKE] Don't ship the AUR package sources as part of the release

Summary:
They are not useful for building the node software, so exclude them from
the source package.

Test Plan:

ninja package_source

Check the aur directory is not part of the source archive.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7407