autopkgtest: testbed is trimmed down too much

Hi,

Running https://debusine.freexian.com/freexian/elts/work-request/16076/#work_request,
https://debusine.freexian.com/freexian/elts/artifact/673084/file/log

35s /tmp/autopkgtest.6wqhk9/wrapper.sh: 233: /tmp/autopkgtest.6wqhk9/wrapper.sh: ps: not found

This is from https://salsa.debian.org/ci-team/autopkgtest/-/blob/debian/bookworm-backports/lib/in-testbed/wrapper.sh?ref_type=heads#L233 .

It looks like procps is not installed in the testbed:
https://debusine.freexian.com/freexian/base/artifact/672702/#metadata
so configured a bit too strictly. This appears to be "tarball:buster:amd64::incus-lxc".

There is also a WARNING which may also be related trimming too much:

autopkgtest: WARNING: Failed to get the testbed release: Could not guess the default release from the APT sources.