Update Coverity Scan CI job to 2020.09

Also make sure that a check for cov-analysis-linux64-2020.09 directory being in place is more verbose.

Coverity CI job failed without much of information: https://gitlab.isc.org/isc-projects/bind9/-/jobs/1426368

Coverity CI job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/1427740

Closes #2415 (closed)