[9.20] chg: ci: Re-enable shotgun runs for nightlies and tags

The recent rewrite of DNS Shotgun infrastructure might've improved the
prior instability. In order to evaluate, re-enable the regular shotgun
pipelines to gather data.

Backport of MR !11506

Merge branch 'backport-nicki/ci-shotgun-enable-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11644