Build models in repository as part of CI build

[hannobraun]

The CI build doesn't currently build the models in the models/ directory, which means that errors can creep in there unnoticed.

Maybe it could be as simple as creating a Cargo workspace, and adding the models to it. If not, adding a shell script (or ideally something more portable) that goes through everything in models/ and runs cargo build would work.

Labeling as https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, since this is purely a Cargo thing that doesn't require any knowledge of Fornjot.

[hannobraun]

If adding a build script is necessary to address this issue, my musings about that topic in #64 are relevant.