package tests failing

[graphaelli]

eg https://apm-ci.elastic.co/job/elastic+apm-server+master+package/17/console:

./_beats/dev-tools/jenkins_release.sh: line 21: VERSION_QUALIFIER: unbound variable

because we have set -u.

introduced in #1565.

[graphaelli]

a proper fix for this is in elastic/beats#9148. We a little ways off from being able to update libbeat right now due to issues discovered in #1567