* guix/tests/foreign.scm (debian-12-qcow2): rename to debian-13-qcow2.
(debian-13-qcow2): new URL and checksum.
(%test-debian-install): use new variable.
(%test-debian-install): add comment on possible test failures.
Change-Id: I0eb241b5c88f73a822edb20815fe09f8c76a2b6b
Signed-off-by: Efraim Flashner <efraim@flashner.co.il>