[ci] checkout and run the uss-qualifier as part of the CI

[Shastick]

To increase test coverage, we add a step to the CI that will run the qualifier against the DSS version built from the current PR.

This essentially automates the manual process one would follow to validate a PR on the DSS repo.

Note: the current way to get this working in the CI is to run all the existing configurations with configurations.dev.uspace first: if it is run last, the CI seems to fail (at least it did a few times in a row)