fix migration of artifact type for postgres

[pierrotsmnrd]

Fixes #573

Description

This PR ensures a DB migration script is properly run using Postgres

Pull request checklist

• [x] Did you test this change locally?
• [ ] ~Did you update the documentation (if required)?~
• [ ] ~Did you add/update relevant tests for this change (if required)?~ testing this would require to run all the test suite in both standalone mode, and with Postgres as a backend (for example with the docker-compose file at the root of the project). It's a whole piece of work by itself.

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