jenkins: Unstuck buster-swh image build

Currently, due to a recent change to add a new dependency, we had to rebuild the docker image. The build [1] is stuck for unrelated reasons.

Which results in a poor coverage from for example this diff [2].

Somehow, we cannot retrieve the cassandra release keys from download.apache.org. The curl query is stuck from our networks.

Idea from @vsellier: using a mirror of it.

• [1] https://jenkins.softwareheritage.org/job/jenkins-tools/job/swh-jenkins-dockerfiles/1136/console

• [2] D8636

---

Migrated from T4615 (view on Phabricator)