- cli builds won't have PPAs and other external repos
- packages from such repos should be listed in `packages.external` now, so they're not mismatched
- `PACKAGE_LIST_EXTERNAL` (packages.external) only in DESKTOP builds
- this logic may change later, example:
- we might _also_ want pure-Ubuntu desktops, without external repos
- we might _instead_ want external repos in the cli builds
- this way is easy to change, since everything is kept separate and we can change in a single place.
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