The autopkgtest source package uses deprecated skip-not-installable
restriction in its own autopkgtests. The restriction is used to allow
the LXD tests (d/t/lxd) to be skipped on Debian, where lxd is not
readily available, without extra test runtime logic.

Band-aid MR: https://salsa.debian.org/ci-team/autopkgtest/-/merge_requests/644

One way forward is having d/t/lxd install the lxd snap, but before
implementing that we need clarification on the question posed in
#1031648.

Paride