Jenkinsfile: Fix the ospack filename

Fix the build by using consistent names for the ospack after commit 2089f2cc "gitlab-ci: Add GitLab CI pipeline" changed the default name in the recipe and broke some assumptions in the Jenkinsfile.

Running /debos --artifactdir /scratch/jenkins/workspace/apertis-v2021dev2/images/apertis-lava-build/20200526.0/armhf/nfs --template-var ospack:"ospack-nfs-v2021dev2-armhf.tar.gz" --template-var architecture:"armhf" /scratch/jenkins/workspace/apertis-v2021dev2/images/apertis-lava-build/recipes/apertis-nfs.yaml
2020/05/26 04:06:58 ==== unpack ====
2020/05/26 04:06:58 unpack | tar (child): /scratch/jenkins/workspace/apertis-v2021dev2/images/apertis-lava-build/20200526.0/armhf/nfs/ospack-nfs-v2021dev2-armhf.tar.gz: Cannot open: No such file or directory
2020/05/26 04:06:58 unpack | tar (child): Error is not recoverable: exiting now
2020/05/26 04:06:58 unpack | tar: Child returned status 2
2020/05/26 04:06:58 unpack | tar: Error is not recoverable: exiting now
2020/05/26 04:06:58 Action `unpack` failed at stage Run, error: exit status 2

Test pipeline: https://jenkins.apertis.org/job/playground/job/em/job/apertis-lava-build/2/