Use a dynamic matrix definition in CI

[lilyminium]

Fixes #

Changes made in this Pull Request:

PR Checklist

• [ ] Tests?
• [ ] Docs?
• [ ] CHANGELOG.md updated?
• [ ] AUTHORS.md updated?
• [ ] Issue raised/referenced?

[lilyminium]

time="2023-05-22T06:18:39Z" level=fatal msg="yaml: unmarshal errors:\n line 0: cannot unmarshal !!str \"[\\\"3.9... into []interface {}"

Ok, this made it even stringier than before...