Do not override sources list when rootfs.tarball is used

[Paul Mars]

Currently in u-i the /etc/apt/sources.list.d/ubuntu.sources (or /etc/apt/sources.list) file is systematically overridden after the rootfs creation, whatever the method used (either from a seed or from a tarball). See https://github.com/canonical/ubuntu-image/blob/main/internal/statemachine/classic.go#L354

If no value were given under the rootfs section for components/pocket/mirror, default values are used.

If components/pocket were given in the customization section, then the file is overridden again.

When a tarball is provided, /etc/apt/sources.list.d/ubuntu.sources may already be set to something that should be kept. The current behavior forces u-i users to set the same values in rootfs{.components,.pocket,.mirror} so /etc/apt/sources.list.d/ubuntu.sources can be regenerated with the same content. This is not very practical because users have to maintain the content of /etc/apt/sources.list.d/ubuntu.sources in several places.

Possible solution: we could avoid calling customizeSourcesListState if the tarball rootfs was used.