Unstuck debian package for swh-storage

It fails with missing public key.

08:42:44  Err:6 https://downloads.apache.org/cassandra/debian 40x InRelease
08:42:44    The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 50EE103D162C5A55
08:42:44  Get:8 http://incoming.debian.org/debian-buildd buildd-unstable/main amd64 Packages [199 kB]
08:42:45  Reading package lists...
08:42:45  W: GPG error: https://downloads.apache.org/cassandra/debian 40x InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 50EE103D162C5A55
08:42:45  E: The repository 'http://www.apache.org/dist/cassandra/debian 40x InRelease' is not signed.
08:42:45  E: apt-get update failed

• [1] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/380/console

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Migrated from T3578 (view on Phabricator)