The license.free field is set to false for non-free packages, for free
packages it is set to true or is missing entirely. Thus we need to
default the case of a missing field to true, not false.
The `ci.nix` file contains a number of improvements over
`buildable.nix`. For example, it
- honors `recurseForDerivations` to also build packages inside
attributes,
- builds all package outputs, and
- separates buildable and cacheable outputs such that packages having
`preferLocalBuild` are not cached.
Robert Helgessonandzimbatm