rlFetchSrcForInstalled is very resourceful and I'm afraid it may find
and download a SRPM of the matching NVR from other sources, not
necessarily the same we want to test. Since we know the CI tests
a scratch build hosted in a local repo called "test-artifacts" then
using "dnf download" directly and limited to that specific repo is
a better approach.
I'm adding a fallback using the repo's local files just in case "dnf"
suffers some temporary hiccup.
