[9.20] chg: ci: Use a small always-on runner for lightweight CI jobs

Some CI jobs spend more time pulling the docker image and setting up their environment than running the script. This MR adds a tag `smalljob` to jobs considered lightweight in order to run them on a dedicated small VM.

Backport of MR !11266

Merge branch 'backport-andoni/try-small-vm-for-lightweight-ci-jobs-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11481