fix(setup-testbed): avoid deprecated cloud-config apt_preserve_sources_list
Use supported cloud-config key apt: {preserve_sources_list: true} instead.
setup-testbed previously wrote deprecated apt_preserve_sources_list to /etc/cloud/cloud.cfg.d/01-autopkg.cfg in any image which has cloud-init installed.
If invoked in autopkgtests, the cloud-init status command exited non-zero (2) indicating a warning when it encountered the deprecated key.
The cloud-init status warning previously emitted are below.
recoverable_errors: DEPRECATED:
- 'The following config key(s): [''apt_preserve_sources_list''] is deprecated in 22.1 and scheduled to be removed in 27.1.'
- Support for combined old and new apt module keys is deprecated in 22.1 and scheduled to be removed in 27.1.