Collect TXT and HTML reports produced by ABI checker
Although in util/api-checker.sh we create textual reports, we don't
preserve them in job artifacts, but we should.
We don't want to keep all HTML pages present in the project root, but just those produced by ABI checker.
A recent job manifesting this limitations: https://gitlab.isc.org/isc-private/bind9/-/jobs/868967.