[9.18] chg: ci: Temporarily disable shotgun jobs

There's currently an issue with the shotgun workflow that's being
investigated. Until it's resolved, there's no point in creating the
shotgun jobs as they'll just fail.

Backport of MR !11005

Merge branch 'backport-nicki/ci-temporarily-disable-shotgun-jobs-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11009