Backport CI fixes to v2_6

Fix CI jobs that are preventing MRs in v2_6:

#3497 (closed) https://gitlab.isc.org/isc-projects/kea/-/jobs/4514644
shellcheck failing because it is running on updated image with newer shellcheck without the appeased warnings https://gitlab.isc.org/isc-projects/kea/-/jobs/4514643

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