Use srun and always provide artifact of benchmark

.gitlab-ci.yml

@@ -39,6 +39,7 @@ test_JUWELS:
     - login
     - shell
   artifacts:
+    when: always
     paths:
       - benchmarks
       - sbatch.err

etc/juwels_benchmark.sh

@@ -6,4 +6,4 @@
 #SBATCH --output=sbatch.out
 #SBATCH --error=sbatch.err
 
-python -m pytest --continue-on-collection-errors -v pySDC/tests -m "benchmark" --benchmark-json=benchmarks/output.json
+srun python -m pytest --continue-on-collection-errors -v pySDC/tests -m "benchmark" --benchmark-json=benchmarks/output.json