Let assets-link also consume files

Passing variables between jobs is not possible with GitLab. Instead, we have to store these in a file, and then use those as an artifact.

By foregoing the whole concept of trying to shoehorn a json file into a string (which is simple, granted, by just using --assets-link $(cat assets.json), instead just allow us to consume the file immediately.

The --description argument can take either a text string, or a file. The same should work for asset-links.